core/cell.rs
1//! Shareable mutable containers.
2//!
3//! Rust memory safety is based on this rule: Given an object `T`, it is only possible to
4//! have one of the following:
5//!
6//! - Several immutable references (`&T`) to the object (also known as **aliasing**).
7//! - One mutable reference (`&mut T`) to the object (also known as **mutability**).
8//!
9//! This is enforced by the Rust compiler. However, there are situations where this rule is not
10//! flexible enough. Sometimes it is required to have multiple references to an object and yet
11//! mutate it.
12//!
13//! Shareable mutable containers exist to permit mutability in a controlled manner, even in the
14//! presence of aliasing. [`Cell<T>`], [`RefCell<T>`], and [`OnceCell<T>`] allow doing this in
15//! a single-threaded way—they do not implement [`Sync`]. (If you need to do aliasing and
16//! mutation among multiple threads, [`Mutex<T>`], [`RwLock<T>`], [`OnceLock<T>`] or [`atomic`]
17//! types are the correct data structures to do so).
18//!
19//! Values of the `Cell<T>`, `RefCell<T>`, and `OnceCell<T>` types may be mutated through shared
20//! references (i.e. the common `&T` type), whereas most Rust types can only be mutated through
21//! unique (`&mut T`) references. We say these cell types provide 'interior mutability'
22//! (mutable via `&T`), in contrast with typical Rust types that exhibit 'inherited mutability'
23//! (mutable only via `&mut T`).
24//!
25//! Cell types come in four flavors: `Cell<T>`, `RefCell<T>`, `OnceCell<T>`, and `LazyCell<T>`.
26//! Each provides a different way of providing safe interior mutability.
27//!
28//! ## `Cell<T>`
29//!
30//! [`Cell<T>`] implements interior mutability by moving values in and out of the cell. That is, an
31//! `&mut T` to the inner value can never be obtained, and the value itself cannot be directly
32//! obtained without replacing it with something else. Both of these rules ensure that there is
33//! never more than one reference pointing to the inner value. This type provides the following
34//! methods:
35//!
36//! - For types that implement [`Copy`], the [`get`](Cell::get) method retrieves the current
37//! interior value by duplicating it.
38//! - For types that implement [`Default`], the [`take`](Cell::take) method replaces the current
39//! interior value with [`Default::default()`] and returns the replaced value.
40//! - All types have:
41//! - [`replace`](Cell::replace): replaces the current interior value and returns the replaced
42//! value.
43//! - [`into_inner`](Cell::into_inner): this method consumes the `Cell<T>` and returns the
44//! interior value.
45//! - [`set`](Cell::set): this method replaces the interior value, dropping the replaced value.
46//!
47//! `Cell<T>` is typically used for more simple types where copying or moving values isn't too
48//! resource intensive (e.g. numbers), and should usually be preferred over other cell types when
49//! possible. For larger and non-copy types, `RefCell` provides some advantages.
50//!
51//! ## `RefCell<T>`
52//!
53//! [`RefCell<T>`] uses Rust's lifetimes to implement "dynamic borrowing", a process whereby one can
54//! claim temporary, exclusive, mutable access to the inner value. Borrows for `RefCell<T>`s are
55//! tracked at _runtime_, unlike Rust's native reference types which are entirely tracked
56//! statically, at compile time.
57//!
58//! An immutable reference to a `RefCell`'s inner value (`&T`) can be obtained with
59//! [`borrow`](`RefCell::borrow`), and a mutable borrow (`&mut T`) can be obtained with
60//! [`borrow_mut`](`RefCell::borrow_mut`). When these functions are called, they first verify that
61//! Rust's borrow rules will be satisfied: any number of immutable borrows are allowed or a
62//! single mutable borrow is allowed, but never both. If a borrow is attempted that would violate
63//! these rules, the thread will panic.
64//!
65//! The corresponding [`Sync`] version of `RefCell<T>` is [`RwLock<T>`].
66//!
67//! ## `OnceCell<T>`
68//!
69//! [`OnceCell<T>`] is somewhat of a hybrid of `Cell` and `RefCell` that works for values that
70//! typically only need to be set once. This means that a reference `&T` can be obtained without
71//! moving or copying the inner value (unlike `Cell`) but also without runtime checks (unlike
72//! `RefCell`). However, its value can also not be updated once set unless you have a mutable
73//! reference to the `OnceCell`.
74//!
75//! `OnceCell` provides the following methods:
76//!
77//! - [`get`](OnceCell::get): obtain a reference to the inner value
78//! - [`set`](OnceCell::set): set the inner value if it is unset (returns a `Result`)
79//! - [`get_or_init`](OnceCell::get_or_init): return the inner value, initializing it if needed
80//! - [`get_mut`](OnceCell::get_mut): provide a mutable reference to the inner value, only available
81//! if you have a mutable reference to the cell itself.
82//!
83//! The corresponding [`Sync`] version of `OnceCell<T>` is [`OnceLock<T>`].
84//!
85//! ## `LazyCell<T, F>`
86//!
87//! A common pattern with OnceCell is, for a given OnceCell, to use the same function on every
88//! call to [`OnceCell::get_or_init`] with that cell. This is what is offered by [`LazyCell`],
89//! which pairs cells of `T` with functions of `F`, and always calls `F` before it yields `&T`.
90//! This happens implicitly by simply attempting to dereference the LazyCell to get its contents,
91//! so its use is much more transparent with a place which has been initialized by a constant.
92//!
93//! More complicated patterns that don't fit this description can be built on `OnceCell<T>` instead.
94//!
95//! `LazyCell` works by providing an implementation of `impl Deref` that calls the function,
96//! so you can just use it by dereference (e.g. `*lazy_cell` or `lazy_cell.deref()`).
97//!
98//! The corresponding [`Sync`] version of `LazyCell<T, F>` is [`LazyLock<T, F>`].
99//!
100//! # When to choose interior mutability
101//!
102//! The more common inherited mutability, where one must have unique access to mutate a value, is
103//! one of the key language elements that enables Rust to reason strongly about pointer aliasing,
104//! statically preventing crash bugs. Because of that, inherited mutability is preferred, and
105//! interior mutability is something of a last resort. Since cell types enable mutation where it
106//! would otherwise be disallowed though, there are occasions when interior mutability might be
107//! appropriate, or even *must* be used, e.g.
108//!
109//! * Introducing mutability 'inside' of something immutable
110//! * Implementation details of logically-immutable methods.
111//! * Mutating implementations of [`Clone`].
112//!
113//! ## Introducing mutability 'inside' of something immutable
114//!
115//! Many shared smart pointer types, including [`Rc<T>`] and [`Arc<T>`], provide containers that can
116//! be cloned and shared between multiple parties. Because the contained values may be
117//! multiply-aliased, they can only be borrowed with `&`, not `&mut`. Without cells it would be
118//! impossible to mutate data inside of these smart pointers at all.
119//!
120//! It's very common then to put a `RefCell<T>` inside shared pointer types to reintroduce
121//! mutability:
122//!
123//! ```
124//! use std::cell::{RefCell, RefMut};
125//! use std::collections::HashMap;
126//! use std::rc::Rc;
127//!
128//! fn main() {
129//! let shared_map: Rc<RefCell<_>> = Rc::new(RefCell::new(HashMap::new()));
130//! // Create a new block to limit the scope of the dynamic borrow
131//! {
132//! let mut map: RefMut<'_, _> = shared_map.borrow_mut();
133//! map.insert("africa", 92388);
134//! map.insert("kyoto", 11837);
135//! map.insert("piccadilly", 11826);
136//! map.insert("marbles", 38);
137//! }
138//!
139//! // Note that if we had not let the previous borrow of the cache fall out
140//! // of scope then the subsequent borrow would cause a dynamic thread panic.
141//! // This is the major hazard of using `RefCell`.
142//! let total: i32 = shared_map.borrow().values().sum();
143//! println!("{total}");
144//! }
145//! ```
146//!
147//! Note that this example uses `Rc<T>` and not `Arc<T>`. `RefCell<T>`s are for single-threaded
148//! scenarios. Consider using [`RwLock<T>`] or [`Mutex<T>`] if you need shared mutability in a
149//! multi-threaded situation.
150//!
151//! ## Implementation details of logically-immutable methods
152//!
153//! Occasionally it may be desirable not to expose in an API that there is mutation happening
154//! "under the hood". This may be because logically the operation is immutable, but e.g., caching
155//! forces the implementation to perform mutation; or because you must employ mutation to implement
156//! a trait method that was originally defined to take `&self`.
157//!
158//! ```
159//! # #![allow(dead_code)]
160//! use std::cell::OnceCell;
161//!
162//! struct Graph {
163//! edges: Vec<(i32, i32)>,
164//! span_tree_cache: OnceCell<Vec<(i32, i32)>>
165//! }
166//!
167//! impl Graph {
168//! fn minimum_spanning_tree(&self) -> Vec<(i32, i32)> {
169//! self.span_tree_cache
170//! .get_or_init(|| self.calc_span_tree())
171//! .clone()
172//! }
173//!
174//! fn calc_span_tree(&self) -> Vec<(i32, i32)> {
175//! // Expensive computation goes here
176//! vec![]
177//! }
178//! }
179//! ```
180//!
181//! ## Mutating implementations of `Clone`
182//!
183//! This is simply a special - but common - case of the previous: hiding mutability for operations
184//! that appear to be immutable. The [`clone`](Clone::clone) method is expected to not change the
185//! source value, and is declared to take `&self`, not `&mut self`. Therefore, any mutation that
186//! happens in the `clone` method must use cell types. For example, [`Rc<T>`] maintains its
187//! reference counts within a `Cell<T>`.
188//!
189//! ```
190//! use std::cell::Cell;
191//! use std::ptr::NonNull;
192//! use std::process::abort;
193//! use std::marker::PhantomData;
194//!
195//! struct Rc<T: ?Sized> {
196//! ptr: NonNull<RcInner<T>>,
197//! phantom: PhantomData<RcInner<T>>,
198//! }
199//!
200//! struct RcInner<T: ?Sized> {
201//! strong: Cell<usize>,
202//! refcount: Cell<usize>,
203//! value: T,
204//! }
205//!
206//! impl<T: ?Sized> Clone for Rc<T> {
207//! fn clone(&self) -> Rc<T> {
208//! self.inc_strong();
209//! Rc {
210//! ptr: self.ptr,
211//! phantom: PhantomData,
212//! }
213//! }
214//! }
215//!
216//! trait RcInnerPtr<T: ?Sized> {
217//!
218//! fn inner(&self) -> &RcInner<T>;
219//!
220//! fn strong(&self) -> usize {
221//! self.inner().strong.get()
222//! }
223//!
224//! fn inc_strong(&self) {
225//! self.inner()
226//! .strong
227//! .set(self.strong()
228//! .checked_add(1)
229//! .unwrap_or_else(|| abort() ));
230//! }
231//! }
232//!
233//! impl<T: ?Sized> RcInnerPtr<T> for Rc<T> {
234//! fn inner(&self) -> &RcInner<T> {
235//! unsafe {
236//! self.ptr.as_ref()
237//! }
238//! }
239//! }
240//! ```
241//!
242//! [`Arc<T>`]: ../../std/sync/struct.Arc.html
243//! [`Rc<T>`]: ../../std/rc/struct.Rc.html
244//! [`RwLock<T>`]: ../../std/sync/struct.RwLock.html
245//! [`Mutex<T>`]: ../../std/sync/struct.Mutex.html
246//! [`OnceLock<T>`]: ../../std/sync/struct.OnceLock.html
247//! [`LazyLock<T, F>`]: ../../std/sync/struct.LazyLock.html
248//! [`Sync`]: ../../std/marker/trait.Sync.html
249//! [`atomic`]: crate::sync::atomic
250
251#![stable(feature = "rust1", since = "1.0.0")]
252
253use crate::cmp::Ordering;
254use crate::fmt::{self, Debug, Display};
255use crate::marker::{PhantomData, Unsize};
256use crate::mem;
257use crate::ops::{CoerceUnsized, Deref, DerefMut, DerefPure, DispatchFromDyn};
258use crate::panic::const_panic;
259use crate::pin::PinCoerceUnsized;
260use crate::ptr::{self, NonNull};
261
262mod lazy;
263mod once;
264
265#[stable(feature = "lazy_cell", since = "1.80.0")]
266pub use lazy::LazyCell;
267#[stable(feature = "once_cell", since = "1.70.0")]
268pub use once::OnceCell;
269
270/// A mutable memory location.
271///
272/// # Memory layout
273///
274/// `Cell<T>` has the same [memory layout and caveats as
275/// `UnsafeCell<T>`](UnsafeCell#memory-layout). In particular, this means that
276/// `Cell<T>` has the same in-memory representation as its inner type `T`.
277///
278/// # Examples
279///
280/// In this example, you can see that `Cell<T>` enables mutation inside an
281/// immutable struct. In other words, it enables "interior mutability".
282///
283/// ```
284/// use std::cell::Cell;
285///
286/// struct SomeStruct {
287/// regular_field: u8,
288/// special_field: Cell<u8>,
289/// }
290///
291/// let my_struct = SomeStruct {
292/// regular_field: 0,
293/// special_field: Cell::new(1),
294/// };
295///
296/// let new_value = 100;
297///
298/// // ERROR: `my_struct` is immutable
299/// // my_struct.regular_field = new_value;
300///
301/// // WORKS: although `my_struct` is immutable, `special_field` is a `Cell`,
302/// // which can always be mutated
303/// my_struct.special_field.set(new_value);
304/// assert_eq!(my_struct.special_field.get(), new_value);
305/// ```
306///
307/// See the [module-level documentation](self) for more.
308#[rustc_diagnostic_item = "Cell"]
309#[stable(feature = "rust1", since = "1.0.0")]
310#[repr(transparent)]
311#[rustc_pub_transparent]
312pub struct Cell<T: ?Sized> {
313 value: UnsafeCell<T>,
314}
315
316#[stable(feature = "rust1", since = "1.0.0")]
317unsafe impl<T: ?Sized> Send for Cell<T> where T: Send {}
318
319// Note that this negative impl isn't strictly necessary for correctness,
320// as `Cell` wraps `UnsafeCell`, which is itself `!Sync`.
321// However, given how important `Cell`'s `!Sync`-ness is,
322// having an explicit negative impl is nice for documentation purposes
323// and results in nicer error messages.
324#[stable(feature = "rust1", since = "1.0.0")]
325impl<T: ?Sized> !Sync for Cell<T> {}
326
327#[stable(feature = "rust1", since = "1.0.0")]
328impl<T: Copy> Clone for Cell<T> {
329 #[inline]
330 fn clone(&self) -> Cell<T> {
331 Cell::new(self.get())
332 }
333}
334
335#[stable(feature = "rust1", since = "1.0.0")]
336impl<T: Default> Default for Cell<T> {
337 /// Creates a `Cell<T>`, with the `Default` value for T.
338 #[inline]
339 fn default() -> Cell<T> {
340 Cell::new(Default::default())
341 }
342}
343
344#[stable(feature = "rust1", since = "1.0.0")]
345impl<T: PartialEq + Copy> PartialEq for Cell<T> {
346 #[inline]
347 fn eq(&self, other: &Cell<T>) -> bool {
348 self.get() == other.get()
349 }
350}
351
352#[stable(feature = "cell_eq", since = "1.2.0")]
353impl<T: Eq + Copy> Eq for Cell<T> {}
354
355#[stable(feature = "cell_ord", since = "1.10.0")]
356impl<T: PartialOrd + Copy> PartialOrd for Cell<T> {
357 #[inline]
358 fn partial_cmp(&self, other: &Cell<T>) -> Option<Ordering> {
359 self.get().partial_cmp(&other.get())
360 }
361
362 #[inline]
363 fn lt(&self, other: &Cell<T>) -> bool {
364 self.get() < other.get()
365 }
366
367 #[inline]
368 fn le(&self, other: &Cell<T>) -> bool {
369 self.get() <= other.get()
370 }
371
372 #[inline]
373 fn gt(&self, other: &Cell<T>) -> bool {
374 self.get() > other.get()
375 }
376
377 #[inline]
378 fn ge(&self, other: &Cell<T>) -> bool {
379 self.get() >= other.get()
380 }
381}
382
383#[stable(feature = "cell_ord", since = "1.10.0")]
384impl<T: Ord + Copy> Ord for Cell<T> {
385 #[inline]
386 fn cmp(&self, other: &Cell<T>) -> Ordering {
387 self.get().cmp(&other.get())
388 }
389}
390
391#[stable(feature = "cell_from", since = "1.12.0")]
392impl<T> From<T> for Cell<T> {
393 /// Creates a new `Cell<T>` containing the given value.
394 fn from(t: T) -> Cell<T> {
395 Cell::new(t)
396 }
397}
398
399impl<T> Cell<T> {
400 /// Creates a new `Cell` containing the given value.
401 ///
402 /// # Examples
403 ///
404 /// ```
405 /// use std::cell::Cell;
406 ///
407 /// let c = Cell::new(5);
408 /// ```
409 #[stable(feature = "rust1", since = "1.0.0")]
410 #[rustc_const_stable(feature = "const_cell_new", since = "1.24.0")]
411 #[inline]
412 pub const fn new(value: T) -> Cell<T> {
413 Cell { value: UnsafeCell::new(value) }
414 }
415
416 /// Sets the contained value.
417 ///
418 /// # Examples
419 ///
420 /// ```
421 /// use std::cell::Cell;
422 ///
423 /// let c = Cell::new(5);
424 ///
425 /// c.set(10);
426 /// ```
427 #[inline]
428 #[stable(feature = "rust1", since = "1.0.0")]
429 pub fn set(&self, val: T) {
430 self.replace(val);
431 }
432
433 /// Swaps the values of two `Cell`s.
434 ///
435 /// The difference with `std::mem::swap` is that this function doesn't
436 /// require a `&mut` reference.
437 ///
438 /// # Panics
439 ///
440 /// This function will panic if `self` and `other` are different `Cell`s that partially overlap.
441 /// (Using just standard library methods, it is impossible to create such partially overlapping `Cell`s.
442 /// However, unsafe code is allowed to e.g. create two `&Cell<[i32; 2]>` that partially overlap.)
443 ///
444 /// # Examples
445 ///
446 /// ```
447 /// use std::cell::Cell;
448 ///
449 /// let c1 = Cell::new(5i32);
450 /// let c2 = Cell::new(10i32);
451 /// c1.swap(&c2);
452 /// assert_eq!(10, c1.get());
453 /// assert_eq!(5, c2.get());
454 /// ```
455 #[inline]
456 #[stable(feature = "move_cell", since = "1.17.0")]
457 pub fn swap(&self, other: &Self) {
458 // This function documents that it *will* panic, and intrinsics::is_nonoverlapping doesn't
459 // do the check in const, so trying to use it here would be inviting unnecessary fragility.
460 fn is_nonoverlapping<T>(src: *const T, dst: *const T) -> bool {
461 let src_usize = src.addr();
462 let dst_usize = dst.addr();
463 let diff = src_usize.abs_diff(dst_usize);
464 diff >= size_of::<T>()
465 }
466
467 if ptr::eq(self, other) {
468 // Swapping wouldn't change anything.
469 return;
470 }
471 if !is_nonoverlapping(self, other) {
472 // See <https://github.com/rust-lang/rust/issues/80778> for why we need to stop here.
473 panic!("`Cell::swap` on overlapping non-identical `Cell`s");
474 }
475 // SAFETY: This can be risky if called from separate threads, but `Cell`
476 // is `!Sync` so this won't happen. This also won't invalidate any
477 // pointers since `Cell` makes sure nothing else will be pointing into
478 // either of these `Cell`s. We also excluded shenanigans like partially overlapping `Cell`s,
479 // so `swap` will just properly copy two full values of type `T` back and forth.
480 unsafe {
481 mem::swap(&mut *self.value.get(), &mut *other.value.get());
482 }
483 }
484
485 /// Replaces the contained value with `val`, and returns the old contained value.
486 ///
487 /// # Examples
488 ///
489 /// ```
490 /// use std::cell::Cell;
491 ///
492 /// let cell = Cell::new(5);
493 /// assert_eq!(cell.get(), 5);
494 /// assert_eq!(cell.replace(10), 5);
495 /// assert_eq!(cell.get(), 10);
496 /// ```
497 #[inline]
498 #[stable(feature = "move_cell", since = "1.17.0")]
499 #[rustc_const_stable(feature = "const_cell", since = "1.88.0")]
500 #[rustc_confusables("swap")]
501 pub const fn replace(&self, val: T) -> T {
502 // SAFETY: This can cause data races if called from a separate thread,
503 // but `Cell` is `!Sync` so this won't happen.
504 mem::replace(unsafe { &mut *self.value.get() }, val)
505 }
506
507 /// Unwraps the value, consuming the cell.
508 ///
509 /// # Examples
510 ///
511 /// ```
512 /// use std::cell::Cell;
513 ///
514 /// let c = Cell::new(5);
515 /// let five = c.into_inner();
516 ///
517 /// assert_eq!(five, 5);
518 /// ```
519 #[stable(feature = "move_cell", since = "1.17.0")]
520 #[rustc_const_stable(feature = "const_cell_into_inner", since = "1.83.0")]
521 #[rustc_allow_const_fn_unstable(const_precise_live_drops)]
522 pub const fn into_inner(self) -> T {
523 self.value.into_inner()
524 }
525}
526
527impl<T: Copy> Cell<T> {
528 /// Returns a copy of the contained value.
529 ///
530 /// # Examples
531 ///
532 /// ```
533 /// use std::cell::Cell;
534 ///
535 /// let c = Cell::new(5);
536 ///
537 /// let five = c.get();
538 /// ```
539 #[inline]
540 #[stable(feature = "rust1", since = "1.0.0")]
541 #[rustc_const_stable(feature = "const_cell", since = "1.88.0")]
542 pub const fn get(&self) -> T {
543 // SAFETY: This can cause data races if called from a separate thread,
544 // but `Cell` is `!Sync` so this won't happen.
545 unsafe { *self.value.get() }
546 }
547
548 /// Updates the contained value using a function.
549 ///
550 /// # Examples
551 ///
552 /// ```
553 /// use std::cell::Cell;
554 ///
555 /// let c = Cell::new(5);
556 /// c.update(|x| x + 1);
557 /// assert_eq!(c.get(), 6);
558 /// ```
559 #[inline]
560 #[stable(feature = "cell_update", since = "1.88.0")]
561 pub fn update(&self, f: impl FnOnce(T) -> T) {
562 let old = self.get();
563 self.set(f(old));
564 }
565}
566
567impl<T: ?Sized> Cell<T> {
568 /// Returns a raw pointer to the underlying data in this cell.
569 ///
570 /// # Examples
571 ///
572 /// ```
573 /// use std::cell::Cell;
574 ///
575 /// let c = Cell::new(5);
576 ///
577 /// let ptr = c.as_ptr();
578 /// ```
579 #[inline]
580 #[stable(feature = "cell_as_ptr", since = "1.12.0")]
581 #[rustc_const_stable(feature = "const_cell_as_ptr", since = "1.32.0")]
582 #[rustc_as_ptr]
583 #[rustc_never_returns_null_ptr]
584 pub const fn as_ptr(&self) -> *mut T {
585 self.value.get()
586 }
587
588 /// Returns a mutable reference to the underlying data.
589 ///
590 /// This call borrows `Cell` mutably (at compile-time) which guarantees
591 /// that we possess the only reference.
592 ///
593 /// However be cautious: this method expects `self` to be mutable, which is
594 /// generally not the case when using a `Cell`. If you require interior
595 /// mutability by reference, consider using `RefCell` which provides
596 /// run-time checked mutable borrows through its [`borrow_mut`] method.
597 ///
598 /// [`borrow_mut`]: RefCell::borrow_mut()
599 ///
600 /// # Examples
601 ///
602 /// ```
603 /// use std::cell::Cell;
604 ///
605 /// let mut c = Cell::new(5);
606 /// *c.get_mut() += 1;
607 ///
608 /// assert_eq!(c.get(), 6);
609 /// ```
610 #[inline]
611 #[stable(feature = "cell_get_mut", since = "1.11.0")]
612 #[rustc_const_stable(feature = "const_cell", since = "1.88.0")]
613 pub const fn get_mut(&mut self) -> &mut T {
614 self.value.get_mut()
615 }
616
617 /// Returns a `&Cell<T>` from a `&mut T`
618 ///
619 /// # Examples
620 ///
621 /// ```
622 /// use std::cell::Cell;
623 ///
624 /// let slice: &mut [i32] = &mut [1, 2, 3];
625 /// let cell_slice: &Cell<[i32]> = Cell::from_mut(slice);
626 /// let slice_cell: &[Cell<i32>] = cell_slice.as_slice_of_cells();
627 ///
628 /// assert_eq!(slice_cell.len(), 3);
629 /// ```
630 #[inline]
631 #[stable(feature = "as_cell", since = "1.37.0")]
632 #[rustc_const_stable(feature = "const_cell", since = "1.88.0")]
633 pub const fn from_mut(t: &mut T) -> &Cell<T> {
634 // SAFETY: `&mut` ensures unique access.
635 unsafe { &*(t as *mut T as *const Cell<T>) }
636 }
637}
638
639impl<T: Default> Cell<T> {
640 /// Takes the value of the cell, leaving `Default::default()` in its place.
641 ///
642 /// # Examples
643 ///
644 /// ```
645 /// use std::cell::Cell;
646 ///
647 /// let c = Cell::new(5);
648 /// let five = c.take();
649 ///
650 /// assert_eq!(five, 5);
651 /// assert_eq!(c.into_inner(), 0);
652 /// ```
653 #[stable(feature = "move_cell", since = "1.17.0")]
654 pub fn take(&self) -> T {
655 self.replace(Default::default())
656 }
657}
658
659#[unstable(feature = "coerce_unsized", issue = "18598")]
660impl<T: CoerceUnsized<U>, U> CoerceUnsized<Cell<U>> for Cell<T> {}
661
662// Allow types that wrap `Cell` to also implement `DispatchFromDyn`
663// and become dyn-compatible method receivers.
664// Note that currently `Cell` itself cannot be a method receiver
665// because it does not implement Deref.
666// In other words:
667// `self: Cell<&Self>` won't work
668// `self: CellWrapper<Self>` becomes possible
669#[unstable(feature = "dispatch_from_dyn", issue = "none")]
670impl<T: DispatchFromDyn<U>, U> DispatchFromDyn<Cell<U>> for Cell<T> {}
671
672impl<T> Cell<[T]> {
673 /// Returns a `&[Cell<T>]` from a `&Cell<[T]>`
674 ///
675 /// # Examples
676 ///
677 /// ```
678 /// use std::cell::Cell;
679 ///
680 /// let slice: &mut [i32] = &mut [1, 2, 3];
681 /// let cell_slice: &Cell<[i32]> = Cell::from_mut(slice);
682 /// let slice_cell: &[Cell<i32>] = cell_slice.as_slice_of_cells();
683 ///
684 /// assert_eq!(slice_cell.len(), 3);
685 /// ```
686 #[stable(feature = "as_cell", since = "1.37.0")]
687 #[rustc_const_stable(feature = "const_cell", since = "1.88.0")]
688 pub const fn as_slice_of_cells(&self) -> &[Cell<T>] {
689 // SAFETY: `Cell<T>` has the same memory layout as `T`.
690 unsafe { &*(self as *const Cell<[T]> as *const [Cell<T>]) }
691 }
692}
693
694impl<T, const N: usize> Cell<[T; N]> {
695 /// Returns a `&[Cell<T>; N]` from a `&Cell<[T; N]>`
696 ///
697 /// # Examples
698 ///
699 /// ```
700 /// #![feature(as_array_of_cells)]
701 /// use std::cell::Cell;
702 ///
703 /// let mut array: [i32; 3] = [1, 2, 3];
704 /// let cell_array: &Cell<[i32; 3]> = Cell::from_mut(&mut array);
705 /// let array_cell: &[Cell<i32>; 3] = cell_array.as_array_of_cells();
706 /// ```
707 #[unstable(feature = "as_array_of_cells", issue = "88248")]
708 pub const fn as_array_of_cells(&self) -> &[Cell<T>; N] {
709 // SAFETY: `Cell<T>` has the same memory layout as `T`.
710 unsafe { &*(self as *const Cell<[T; N]> as *const [Cell<T>; N]) }
711 }
712}
713
714/// A mutable memory location with dynamically checked borrow rules
715///
716/// See the [module-level documentation](self) for more.
717#[rustc_diagnostic_item = "RefCell"]
718#[stable(feature = "rust1", since = "1.0.0")]
719pub struct RefCell<T: ?Sized> {
720 borrow: Cell<BorrowCounter>,
721 // Stores the location of the earliest currently active borrow.
722 // This gets updated whenever we go from having zero borrows
723 // to having a single borrow. When a borrow occurs, this gets included
724 // in the generated `BorrowError`/`BorrowMutError`
725 #[cfg(feature = "debug_refcell")]
726 borrowed_at: Cell<Option<&'static crate::panic::Location<'static>>>,
727 value: UnsafeCell<T>,
728}
729
730/// An error returned by [`RefCell::try_borrow`].
731#[stable(feature = "try_borrow", since = "1.13.0")]
732#[non_exhaustive]
733#[derive(Debug)]
734pub struct BorrowError {
735 #[cfg(feature = "debug_refcell")]
736 location: &'static crate::panic::Location<'static>,
737}
738
739#[stable(feature = "try_borrow", since = "1.13.0")]
740impl Display for BorrowError {
741 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
742 #[cfg(feature = "debug_refcell")]
743 let res = write!(
744 f,
745 "RefCell already mutably borrowed; a previous borrow was at {}",
746 self.location
747 );
748
749 #[cfg(not(feature = "debug_refcell"))]
750 let res = Display::fmt("RefCell already mutably borrowed", f);
751
752 res
753 }
754}
755
756/// An error returned by [`RefCell::try_borrow_mut`].
757#[stable(feature = "try_borrow", since = "1.13.0")]
758#[non_exhaustive]
759#[derive(Debug)]
760pub struct BorrowMutError {
761 #[cfg(feature = "debug_refcell")]
762 location: &'static crate::panic::Location<'static>,
763}
764
765#[stable(feature = "try_borrow", since = "1.13.0")]
766impl Display for BorrowMutError {
767 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
768 #[cfg(feature = "debug_refcell")]
769 let res = write!(f, "RefCell already borrowed; a previous borrow was at {}", self.location);
770
771 #[cfg(not(feature = "debug_refcell"))]
772 let res = Display::fmt("RefCell already borrowed", f);
773
774 res
775 }
776}
777
778// This ensures the panicking code is outlined from `borrow_mut` for `RefCell`.
779#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
780#[track_caller]
781#[cold]
782const fn panic_already_borrowed(err: BorrowMutError) -> ! {
783 const_panic!(
784 "RefCell already borrowed",
785 "{err}",
786 err: BorrowMutError = err,
787 )
788}
789
790// This ensures the panicking code is outlined from `borrow` for `RefCell`.
791#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
792#[track_caller]
793#[cold]
794const fn panic_already_mutably_borrowed(err: BorrowError) -> ! {
795 const_panic!(
796 "RefCell already mutably borrowed",
797 "{err}",
798 err: BorrowError = err,
799 )
800}
801
802// Positive values represent the number of `Ref` active. Negative values
803// represent the number of `RefMut` active. Multiple `RefMut`s can only be
804// active at a time if they refer to distinct, nonoverlapping components of a
805// `RefCell` (e.g., different ranges of a slice).
806//
807// `Ref` and `RefMut` are both two words in size, and so there will likely never
808// be enough `Ref`s or `RefMut`s in existence to overflow half of the `usize`
809// range. Thus, a `BorrowCounter` will probably never overflow or underflow.
810// However, this is not a guarantee, as a pathological program could repeatedly
811// create and then mem::forget `Ref`s or `RefMut`s. Thus, all code must
812// explicitly check for overflow and underflow in order to avoid unsafety, or at
813// least behave correctly in the event that overflow or underflow happens (e.g.,
814// see BorrowRef::new).
815type BorrowCounter = isize;
816const UNUSED: BorrowCounter = 0;
817
818#[inline(always)]
819const fn is_writing(x: BorrowCounter) -> bool {
820 x < UNUSED
821}
822
823#[inline(always)]
824const fn is_reading(x: BorrowCounter) -> bool {
825 x > UNUSED
826}
827
828impl<T> RefCell<T> {
829 /// Creates a new `RefCell` containing `value`.
830 ///
831 /// # Examples
832 ///
833 /// ```
834 /// use std::cell::RefCell;
835 ///
836 /// let c = RefCell::new(5);
837 /// ```
838 #[stable(feature = "rust1", since = "1.0.0")]
839 #[rustc_const_stable(feature = "const_refcell_new", since = "1.24.0")]
840 #[inline]
841 pub const fn new(value: T) -> RefCell<T> {
842 RefCell {
843 value: UnsafeCell::new(value),
844 borrow: Cell::new(UNUSED),
845 #[cfg(feature = "debug_refcell")]
846 borrowed_at: Cell::new(None),
847 }
848 }
849
850 /// Consumes the `RefCell`, returning the wrapped value.
851 ///
852 /// # Examples
853 ///
854 /// ```
855 /// use std::cell::RefCell;
856 ///
857 /// let c = RefCell::new(5);
858 ///
859 /// let five = c.into_inner();
860 /// ```
861 #[stable(feature = "rust1", since = "1.0.0")]
862 #[rustc_const_stable(feature = "const_cell_into_inner", since = "1.83.0")]
863 #[rustc_allow_const_fn_unstable(const_precise_live_drops)]
864 #[inline]
865 pub const fn into_inner(self) -> T {
866 // Since this function takes `self` (the `RefCell`) by value, the
867 // compiler statically verifies that it is not currently borrowed.
868 self.value.into_inner()
869 }
870
871 /// Replaces the wrapped value with a new one, returning the old value,
872 /// without deinitializing either one.
873 ///
874 /// This function corresponds to [`std::mem::replace`](../mem/fn.replace.html).
875 ///
876 /// # Panics
877 ///
878 /// Panics if the value is currently borrowed.
879 ///
880 /// # Examples
881 ///
882 /// ```
883 /// use std::cell::RefCell;
884 /// let cell = RefCell::new(5);
885 /// let old_value = cell.replace(6);
886 /// assert_eq!(old_value, 5);
887 /// assert_eq!(cell, RefCell::new(6));
888 /// ```
889 #[inline]
890 #[stable(feature = "refcell_replace", since = "1.24.0")]
891 #[track_caller]
892 #[rustc_confusables("swap")]
893 #[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
894 pub const fn replace(&self, t: T) -> T {
895 mem::replace(&mut self.borrow_mut(), t)
896 }
897
898 /// Replaces the wrapped value with a new one computed from `f`, returning
899 /// the old value, without deinitializing either one.
900 ///
901 /// # Panics
902 ///
903 /// Panics if the value is currently borrowed.
904 ///
905 /// # Examples
906 ///
907 /// ```
908 /// use std::cell::RefCell;
909 /// let cell = RefCell::new(5);
910 /// let old_value = cell.replace_with(|&mut old| old + 1);
911 /// assert_eq!(old_value, 5);
912 /// assert_eq!(cell, RefCell::new(6));
913 /// ```
914 #[inline]
915 #[stable(feature = "refcell_replace_swap", since = "1.35.0")]
916 #[track_caller]
917 pub fn replace_with<F: FnOnce(&mut T) -> T>(&self, f: F) -> T {
918 let mut_borrow = &mut *self.borrow_mut();
919 let replacement = f(mut_borrow);
920 mem::replace(mut_borrow, replacement)
921 }
922
923 /// Swaps the wrapped value of `self` with the wrapped value of `other`,
924 /// without deinitializing either one.
925 ///
926 /// This function corresponds to [`std::mem::swap`](../mem/fn.swap.html).
927 ///
928 /// # Panics
929 ///
930 /// Panics if the value in either `RefCell` is currently borrowed, or
931 /// if `self` and `other` point to the same `RefCell`.
932 ///
933 /// # Examples
934 ///
935 /// ```
936 /// use std::cell::RefCell;
937 /// let c = RefCell::new(5);
938 /// let d = RefCell::new(6);
939 /// c.swap(&d);
940 /// assert_eq!(c, RefCell::new(6));
941 /// assert_eq!(d, RefCell::new(5));
942 /// ```
943 #[inline]
944 #[stable(feature = "refcell_swap", since = "1.24.0")]
945 #[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
946 pub const fn swap(&self, other: &Self) {
947 mem::swap(&mut *self.borrow_mut(), &mut *other.borrow_mut())
948 }
949}
950
951impl<T: ?Sized> RefCell<T> {
952 /// Immutably borrows the wrapped value.
953 ///
954 /// The borrow lasts until the returned `Ref` exits scope. Multiple
955 /// immutable borrows can be taken out at the same time.
956 ///
957 /// # Panics
958 ///
959 /// Panics if the value is currently mutably borrowed. For a non-panicking variant, use
960 /// [`try_borrow`](#method.try_borrow).
961 ///
962 /// # Examples
963 ///
964 /// ```
965 /// use std::cell::RefCell;
966 ///
967 /// let c = RefCell::new(5);
968 ///
969 /// let borrowed_five = c.borrow();
970 /// let borrowed_five2 = c.borrow();
971 /// ```
972 ///
973 /// An example of panic:
974 ///
975 /// ```should_panic
976 /// use std::cell::RefCell;
977 ///
978 /// let c = RefCell::new(5);
979 ///
980 /// let m = c.borrow_mut();
981 /// let b = c.borrow(); // this causes a panic
982 /// ```
983 #[stable(feature = "rust1", since = "1.0.0")]
984 #[inline]
985 #[track_caller]
986 #[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
987 pub const fn borrow(&self) -> Ref<'_, T> {
988 match self.try_borrow() {
989 Ok(b) => b,
990 Err(err) => panic_already_mutably_borrowed(err),
991 }
992 }
993
994 /// Immutably borrows the wrapped value, returning an error if the value is currently mutably
995 /// borrowed.
996 ///
997 /// The borrow lasts until the returned `Ref` exits scope. Multiple immutable borrows can be
998 /// taken out at the same time.
999 ///
1000 /// This is the non-panicking variant of [`borrow`](#method.borrow).
1001 ///
1002 /// # Examples
1003 ///
1004 /// ```
1005 /// use std::cell::RefCell;
1006 ///
1007 /// let c = RefCell::new(5);
1008 ///
1009 /// {
1010 /// let m = c.borrow_mut();
1011 /// assert!(c.try_borrow().is_err());
1012 /// }
1013 ///
1014 /// {
1015 /// let m = c.borrow();
1016 /// assert!(c.try_borrow().is_ok());
1017 /// }
1018 /// ```
1019 #[stable(feature = "try_borrow", since = "1.13.0")]
1020 #[inline]
1021 #[cfg_attr(feature = "debug_refcell", track_caller)]
1022 #[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
1023 pub const fn try_borrow(&self) -> Result<Ref<'_, T>, BorrowError> {
1024 match BorrowRef::new(&self.borrow) {
1025 Some(b) => {
1026 #[cfg(feature = "debug_refcell")]
1027 {
1028 // `borrowed_at` is always the *first* active borrow
1029 if b.borrow.get() == 1 {
1030 self.borrowed_at.replace(Some(crate::panic::Location::caller()));
1031 }
1032 }
1033
1034 // SAFETY: `BorrowRef` ensures that there is only immutable access
1035 // to the value while borrowed.
1036 let value = unsafe { NonNull::new_unchecked(self.value.get()) };
1037 Ok(Ref { value, borrow: b })
1038 }
1039 None => Err(BorrowError {
1040 // If a borrow occurred, then we must already have an outstanding borrow,
1041 // so `borrowed_at` will be `Some`
1042 #[cfg(feature = "debug_refcell")]
1043 location: self.borrowed_at.get().unwrap(),
1044 }),
1045 }
1046 }
1047
1048 /// Mutably borrows the wrapped value.
1049 ///
1050 /// The borrow lasts until the returned `RefMut` or all `RefMut`s derived
1051 /// from it exit scope. The value cannot be borrowed while this borrow is
1052 /// active.
1053 ///
1054 /// # Panics
1055 ///
1056 /// Panics if the value is currently borrowed. For a non-panicking variant, use
1057 /// [`try_borrow_mut`](#method.try_borrow_mut).
1058 ///
1059 /// # Examples
1060 ///
1061 /// ```
1062 /// use std::cell::RefCell;
1063 ///
1064 /// let c = RefCell::new("hello".to_owned());
1065 ///
1066 /// *c.borrow_mut() = "bonjour".to_owned();
1067 ///
1068 /// assert_eq!(&*c.borrow(), "bonjour");
1069 /// ```
1070 ///
1071 /// An example of panic:
1072 ///
1073 /// ```should_panic
1074 /// use std::cell::RefCell;
1075 ///
1076 /// let c = RefCell::new(5);
1077 /// let m = c.borrow();
1078 ///
1079 /// let b = c.borrow_mut(); // this causes a panic
1080 /// ```
1081 #[stable(feature = "rust1", since = "1.0.0")]
1082 #[inline]
1083 #[track_caller]
1084 #[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
1085 pub const fn borrow_mut(&self) -> RefMut<'_, T> {
1086 match self.try_borrow_mut() {
1087 Ok(b) => b,
1088 Err(err) => panic_already_borrowed(err),
1089 }
1090 }
1091
1092 /// Mutably borrows the wrapped value, returning an error if the value is currently borrowed.
1093 ///
1094 /// The borrow lasts until the returned `RefMut` or all `RefMut`s derived
1095 /// from it exit scope. The value cannot be borrowed while this borrow is
1096 /// active.
1097 ///
1098 /// This is the non-panicking variant of [`borrow_mut`](#method.borrow_mut).
1099 ///
1100 /// # Examples
1101 ///
1102 /// ```
1103 /// use std::cell::RefCell;
1104 ///
1105 /// let c = RefCell::new(5);
1106 ///
1107 /// {
1108 /// let m = c.borrow();
1109 /// assert!(c.try_borrow_mut().is_err());
1110 /// }
1111 ///
1112 /// assert!(c.try_borrow_mut().is_ok());
1113 /// ```
1114 #[stable(feature = "try_borrow", since = "1.13.0")]
1115 #[inline]
1116 #[cfg_attr(feature = "debug_refcell", track_caller)]
1117 #[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
1118 pub const fn try_borrow_mut(&self) -> Result<RefMut<'_, T>, BorrowMutError> {
1119 match BorrowRefMut::new(&self.borrow) {
1120 Some(b) => {
1121 #[cfg(feature = "debug_refcell")]
1122 {
1123 self.borrowed_at.replace(Some(crate::panic::Location::caller()));
1124 }
1125
1126 // SAFETY: `BorrowRefMut` guarantees unique access.
1127 let value = unsafe { NonNull::new_unchecked(self.value.get()) };
1128 Ok(RefMut { value, borrow: b, marker: PhantomData })
1129 }
1130 None => Err(BorrowMutError {
1131 // If a borrow occurred, then we must already have an outstanding borrow,
1132 // so `borrowed_at` will be `Some`
1133 #[cfg(feature = "debug_refcell")]
1134 location: self.borrowed_at.get().unwrap(),
1135 }),
1136 }
1137 }
1138
1139 /// Returns a raw pointer to the underlying data in this cell.
1140 ///
1141 /// # Examples
1142 ///
1143 /// ```
1144 /// use std::cell::RefCell;
1145 ///
1146 /// let c = RefCell::new(5);
1147 ///
1148 /// let ptr = c.as_ptr();
1149 /// ```
1150 #[inline]
1151 #[stable(feature = "cell_as_ptr", since = "1.12.0")]
1152 #[rustc_as_ptr]
1153 #[rustc_never_returns_null_ptr]
1154 #[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
1155 pub const fn as_ptr(&self) -> *mut T {
1156 self.value.get()
1157 }
1158
1159 /// Returns a mutable reference to the underlying data.
1160 ///
1161 /// Since this method borrows `RefCell` mutably, it is statically guaranteed
1162 /// that no borrows to the underlying data exist. The dynamic checks inherent
1163 /// in [`borrow_mut`] and most other methods of `RefCell` are therefore
1164 /// unnecessary. Note that this method does not reset the borrowing state if borrows were previously leaked
1165 /// (e.g., via [`forget()`] on a [`Ref`] or [`RefMut`]). For that purpose,
1166 /// consider using the unstable [`undo_leak`] method.
1167 ///
1168 /// This method can only be called if `RefCell` can be mutably borrowed,
1169 /// which in general is only the case directly after the `RefCell` has
1170 /// been created. In these situations, skipping the aforementioned dynamic
1171 /// borrowing checks may yield better ergonomics and runtime-performance.
1172 ///
1173 /// In most situations where `RefCell` is used, it can't be borrowed mutably.
1174 /// Use [`borrow_mut`] to get mutable access to the underlying data then.
1175 ///
1176 /// [`borrow_mut`]: RefCell::borrow_mut()
1177 /// [`forget()`]: mem::forget
1178 /// [`undo_leak`]: RefCell::undo_leak()
1179 ///
1180 /// # Examples
1181 ///
1182 /// ```
1183 /// use std::cell::RefCell;
1184 ///
1185 /// let mut c = RefCell::new(5);
1186 /// *c.get_mut() += 1;
1187 ///
1188 /// assert_eq!(c, RefCell::new(6));
1189 /// ```
1190 #[inline]
1191 #[stable(feature = "cell_get_mut", since = "1.11.0")]
1192 #[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
1193 pub const fn get_mut(&mut self) -> &mut T {
1194 self.value.get_mut()
1195 }
1196
1197 /// Undo the effect of leaked guards on the borrow state of the `RefCell`.
1198 ///
1199 /// This call is similar to [`get_mut`] but more specialized. It borrows `RefCell` mutably to
1200 /// ensure no borrows exist and then resets the state tracking shared borrows. This is relevant
1201 /// if some `Ref` or `RefMut` borrows have been leaked.
1202 ///
1203 /// [`get_mut`]: RefCell::get_mut()
1204 ///
1205 /// # Examples
1206 ///
1207 /// ```
1208 /// #![feature(cell_leak)]
1209 /// use std::cell::RefCell;
1210 ///
1211 /// let mut c = RefCell::new(0);
1212 /// std::mem::forget(c.borrow_mut());
1213 ///
1214 /// assert!(c.try_borrow().is_err());
1215 /// c.undo_leak();
1216 /// assert!(c.try_borrow().is_ok());
1217 /// ```
1218 #[unstable(feature = "cell_leak", issue = "69099")]
1219 #[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
1220 pub const fn undo_leak(&mut self) -> &mut T {
1221 *self.borrow.get_mut() = UNUSED;
1222 self.get_mut()
1223 }
1224
1225 /// Immutably borrows the wrapped value, returning an error if the value is
1226 /// currently mutably borrowed.
1227 ///
1228 /// # Safety
1229 ///
1230 /// Unlike `RefCell::borrow`, this method is unsafe because it does not
1231 /// return a `Ref`, thus leaving the borrow flag untouched. Mutably
1232 /// borrowing the `RefCell` while the reference returned by this method
1233 /// is alive is undefined behavior.
1234 ///
1235 /// # Examples
1236 ///
1237 /// ```
1238 /// use std::cell::RefCell;
1239 ///
1240 /// let c = RefCell::new(5);
1241 ///
1242 /// {
1243 /// let m = c.borrow_mut();
1244 /// assert!(unsafe { c.try_borrow_unguarded() }.is_err());
1245 /// }
1246 ///
1247 /// {
1248 /// let m = c.borrow();
1249 /// assert!(unsafe { c.try_borrow_unguarded() }.is_ok());
1250 /// }
1251 /// ```
1252 #[stable(feature = "borrow_state", since = "1.37.0")]
1253 #[inline]
1254 #[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
1255 pub const unsafe fn try_borrow_unguarded(&self) -> Result<&T, BorrowError> {
1256 if !is_writing(self.borrow.get()) {
1257 // SAFETY: We check that nobody is actively writing now, but it is
1258 // the caller's responsibility to ensure that nobody writes until
1259 // the returned reference is no longer in use.
1260 // Also, `self.value.get()` refers to the value owned by `self`
1261 // and is thus guaranteed to be valid for the lifetime of `self`.
1262 Ok(unsafe { &*self.value.get() })
1263 } else {
1264 Err(BorrowError {
1265 // If a borrow occurred, then we must already have an outstanding borrow,
1266 // so `borrowed_at` will be `Some`
1267 #[cfg(feature = "debug_refcell")]
1268 location: self.borrowed_at.get().unwrap(),
1269 })
1270 }
1271 }
1272}
1273
1274impl<T: Default> RefCell<T> {
1275 /// Takes the wrapped value, leaving `Default::default()` in its place.
1276 ///
1277 /// # Panics
1278 ///
1279 /// Panics if the value is currently borrowed.
1280 ///
1281 /// # Examples
1282 ///
1283 /// ```
1284 /// use std::cell::RefCell;
1285 ///
1286 /// let c = RefCell::new(5);
1287 /// let five = c.take();
1288 ///
1289 /// assert_eq!(five, 5);
1290 /// assert_eq!(c.into_inner(), 0);
1291 /// ```
1292 #[stable(feature = "refcell_take", since = "1.50.0")]
1293 pub fn take(&self) -> T {
1294 self.replace(Default::default())
1295 }
1296}
1297
1298#[stable(feature = "rust1", since = "1.0.0")]
1299unsafe impl<T: ?Sized> Send for RefCell<T> where T: Send {}
1300
1301#[stable(feature = "rust1", since = "1.0.0")]
1302impl<T: ?Sized> !Sync for RefCell<T> {}
1303
1304#[stable(feature = "rust1", since = "1.0.0")]
1305impl<T: Clone> Clone for RefCell<T> {
1306 /// # Panics
1307 ///
1308 /// Panics if the value is currently mutably borrowed.
1309 #[inline]
1310 #[track_caller]
1311 fn clone(&self) -> RefCell<T> {
1312 RefCell::new(self.borrow().clone())
1313 }
1314
1315 /// # Panics
1316 ///
1317 /// Panics if `source` is currently mutably borrowed.
1318 #[inline]
1319 #[track_caller]
1320 fn clone_from(&mut self, source: &Self) {
1321 self.get_mut().clone_from(&source.borrow())
1322 }
1323}
1324
1325#[stable(feature = "rust1", since = "1.0.0")]
1326impl<T: Default> Default for RefCell<T> {
1327 /// Creates a `RefCell<T>`, with the `Default` value for T.
1328 #[inline]
1329 fn default() -> RefCell<T> {
1330 RefCell::new(Default::default())
1331 }
1332}
1333
1334#[stable(feature = "rust1", since = "1.0.0")]
1335impl<T: ?Sized + PartialEq> PartialEq for RefCell<T> {
1336 /// # Panics
1337 ///
1338 /// Panics if the value in either `RefCell` is currently mutably borrowed.
1339 #[inline]
1340 fn eq(&self, other: &RefCell<T>) -> bool {
1341 *self.borrow() == *other.borrow()
1342 }
1343}
1344
1345#[stable(feature = "cell_eq", since = "1.2.0")]
1346impl<T: ?Sized + Eq> Eq for RefCell<T> {}
1347
1348#[stable(feature = "cell_ord", since = "1.10.0")]
1349impl<T: ?Sized + PartialOrd> PartialOrd for RefCell<T> {
1350 /// # Panics
1351 ///
1352 /// Panics if the value in either `RefCell` is currently mutably borrowed.
1353 #[inline]
1354 fn partial_cmp(&self, other: &RefCell<T>) -> Option<Ordering> {
1355 self.borrow().partial_cmp(&*other.borrow())
1356 }
1357
1358 /// # Panics
1359 ///
1360 /// Panics if the value in either `RefCell` is currently mutably borrowed.
1361 #[inline]
1362 fn lt(&self, other: &RefCell<T>) -> bool {
1363 *self.borrow() < *other.borrow()
1364 }
1365
1366 /// # Panics
1367 ///
1368 /// Panics if the value in either `RefCell` is currently mutably borrowed.
1369 #[inline]
1370 fn le(&self, other: &RefCell<T>) -> bool {
1371 *self.borrow() <= *other.borrow()
1372 }
1373
1374 /// # Panics
1375 ///
1376 /// Panics if the value in either `RefCell` is currently mutably borrowed.
1377 #[inline]
1378 fn gt(&self, other: &RefCell<T>) -> bool {
1379 *self.borrow() > *other.borrow()
1380 }
1381
1382 /// # Panics
1383 ///
1384 /// Panics if the value in either `RefCell` is currently mutably borrowed.
1385 #[inline]
1386 fn ge(&self, other: &RefCell<T>) -> bool {
1387 *self.borrow() >= *other.borrow()
1388 }
1389}
1390
1391#[stable(feature = "cell_ord", since = "1.10.0")]
1392impl<T: ?Sized + Ord> Ord for RefCell<T> {
1393 /// # Panics
1394 ///
1395 /// Panics if the value in either `RefCell` is currently mutably borrowed.
1396 #[inline]
1397 fn cmp(&self, other: &RefCell<T>) -> Ordering {
1398 self.borrow().cmp(&*other.borrow())
1399 }
1400}
1401
1402#[stable(feature = "cell_from", since = "1.12.0")]
1403impl<T> From<T> for RefCell<T> {
1404 /// Creates a new `RefCell<T>` containing the given value.
1405 fn from(t: T) -> RefCell<T> {
1406 RefCell::new(t)
1407 }
1408}
1409
1410#[unstable(feature = "coerce_unsized", issue = "18598")]
1411impl<T: CoerceUnsized<U>, U> CoerceUnsized<RefCell<U>> for RefCell<T> {}
1412
1413struct BorrowRef<'b> {
1414 borrow: &'b Cell<BorrowCounter>,
1415}
1416
1417impl<'b> BorrowRef<'b> {
1418 #[inline]
1419 const fn new(borrow: &'b Cell<BorrowCounter>) -> Option<BorrowRef<'b>> {
1420 let b = borrow.get().wrapping_add(1);
1421 if !is_reading(b) {
1422 // Incrementing borrow can result in a non-reading value (<= 0) in these cases:
1423 // 1. It was < 0, i.e. there are writing borrows, so we can't allow a read borrow
1424 // due to Rust's reference aliasing rules
1425 // 2. It was isize::MAX (the max amount of reading borrows) and it overflowed
1426 // into isize::MIN (the max amount of writing borrows) so we can't allow
1427 // an additional read borrow because isize can't represent so many read borrows
1428 // (this can only happen if you mem::forget more than a small constant amount of
1429 // `Ref`s, which is not good practice)
1430 None
1431 } else {
1432 // Incrementing borrow can result in a reading value (> 0) in these cases:
1433 // 1. It was = 0, i.e. it wasn't borrowed, and we are taking the first read borrow
1434 // 2. It was > 0 and < isize::MAX, i.e. there were read borrows, and isize
1435 // is large enough to represent having one more read borrow
1436 borrow.replace(b);
1437 Some(BorrowRef { borrow })
1438 }
1439 }
1440}
1441
1442#[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
1443impl const Drop for BorrowRef<'_> {
1444 #[inline]
1445 fn drop(&mut self) {
1446 let borrow = self.borrow.get();
1447 debug_assert!(is_reading(borrow));
1448 self.borrow.replace(borrow - 1);
1449 }
1450}
1451
1452#[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
1453impl const Clone for BorrowRef<'_> {
1454 #[inline]
1455 fn clone(&self) -> Self {
1456 // Since this Ref exists, we know the borrow flag
1457 // is a reading borrow.
1458 let borrow = self.borrow.get();
1459 debug_assert!(is_reading(borrow));
1460 // Prevent the borrow counter from overflowing into
1461 // a writing borrow.
1462 assert!(borrow != BorrowCounter::MAX);
1463 self.borrow.replace(borrow + 1);
1464 BorrowRef { borrow: self.borrow }
1465 }
1466}
1467
1468/// Wraps a borrowed reference to a value in a `RefCell` box.
1469/// A wrapper type for an immutably borrowed value from a `RefCell<T>`.
1470///
1471/// See the [module-level documentation](self) for more.
1472#[stable(feature = "rust1", since = "1.0.0")]
1473#[must_not_suspend = "holding a Ref across suspend points can cause BorrowErrors"]
1474#[rustc_diagnostic_item = "RefCellRef"]
1475pub struct Ref<'b, T: ?Sized + 'b> {
1476 // NB: we use a pointer instead of `&'b T` to avoid `noalias` violations, because a
1477 // `Ref` argument doesn't hold immutability for its whole scope, only until it drops.
1478 // `NonNull` is also covariant over `T`, just like we would have with `&T`.
1479 value: NonNull<T>,
1480 borrow: BorrowRef<'b>,
1481}
1482
1483#[stable(feature = "rust1", since = "1.0.0")]
1484#[rustc_const_unstable(feature = "const_deref", issue = "88955")]
1485impl<T: ?Sized> const Deref for Ref<'_, T> {
1486 type Target = T;
1487
1488 #[inline]
1489 fn deref(&self) -> &T {
1490 // SAFETY: the value is accessible as long as we hold our borrow.
1491 unsafe { self.value.as_ref() }
1492 }
1493}
1494
1495#[unstable(feature = "deref_pure_trait", issue = "87121")]
1496unsafe impl<T: ?Sized> DerefPure for Ref<'_, T> {}
1497
1498impl<'b, T: ?Sized> Ref<'b, T> {
1499 /// Copies a `Ref`.
1500 ///
1501 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1502 ///
1503 /// This is an associated function that needs to be used as
1504 /// `Ref::clone(...)`. A `Clone` implementation or a method would interfere
1505 /// with the widespread use of `r.borrow().clone()` to clone the contents of
1506 /// a `RefCell`.
1507 #[stable(feature = "cell_extras", since = "1.15.0")]
1508 #[must_use]
1509 #[inline]
1510 #[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
1511 pub const fn clone(orig: &Ref<'b, T>) -> Ref<'b, T> {
1512 Ref { value: orig.value, borrow: orig.borrow.clone() }
1513 }
1514
1515 /// Makes a new `Ref` for a component of the borrowed data.
1516 ///
1517 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1518 ///
1519 /// This is an associated function that needs to be used as `Ref::map(...)`.
1520 /// A method would interfere with methods of the same name on the contents
1521 /// of a `RefCell` used through `Deref`.
1522 ///
1523 /// # Examples
1524 ///
1525 /// ```
1526 /// use std::cell::{RefCell, Ref};
1527 ///
1528 /// let c = RefCell::new((5, 'b'));
1529 /// let b1: Ref<'_, (u32, char)> = c.borrow();
1530 /// let b2: Ref<'_, u32> = Ref::map(b1, |t| &t.0);
1531 /// assert_eq!(*b2, 5)
1532 /// ```
1533 #[stable(feature = "cell_map", since = "1.8.0")]
1534 #[inline]
1535 pub fn map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Ref<'b, U>
1536 where
1537 F: FnOnce(&T) -> &U,
1538 {
1539 Ref { value: NonNull::from(f(&*orig)), borrow: orig.borrow }
1540 }
1541
1542 /// Makes a new `Ref` for an optional component of the borrowed data. The
1543 /// original guard is returned as an `Err(..)` if the closure returns
1544 /// `None`.
1545 ///
1546 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1547 ///
1548 /// This is an associated function that needs to be used as
1549 /// `Ref::filter_map(...)`. A method would interfere with methods of the same
1550 /// name on the contents of a `RefCell` used through `Deref`.
1551 ///
1552 /// # Examples
1553 ///
1554 /// ```
1555 /// use std::cell::{RefCell, Ref};
1556 ///
1557 /// let c = RefCell::new(vec![1, 2, 3]);
1558 /// let b1: Ref<'_, Vec<u32>> = c.borrow();
1559 /// let b2: Result<Ref<'_, u32>, _> = Ref::filter_map(b1, |v| v.get(1));
1560 /// assert_eq!(*b2.unwrap(), 2);
1561 /// ```
1562 #[stable(feature = "cell_filter_map", since = "1.63.0")]
1563 #[inline]
1564 pub fn filter_map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Result<Ref<'b, U>, Self>
1565 where
1566 F: FnOnce(&T) -> Option<&U>,
1567 {
1568 match f(&*orig) {
1569 Some(value) => Ok(Ref { value: NonNull::from(value), borrow: orig.borrow }),
1570 None => Err(orig),
1571 }
1572 }
1573
1574 /// Splits a `Ref` into multiple `Ref`s for different components of the
1575 /// borrowed data.
1576 ///
1577 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1578 ///
1579 /// This is an associated function that needs to be used as
1580 /// `Ref::map_split(...)`. A method would interfere with methods of the same
1581 /// name on the contents of a `RefCell` used through `Deref`.
1582 ///
1583 /// # Examples
1584 ///
1585 /// ```
1586 /// use std::cell::{Ref, RefCell};
1587 ///
1588 /// let cell = RefCell::new([1, 2, 3, 4]);
1589 /// let borrow = cell.borrow();
1590 /// let (begin, end) = Ref::map_split(borrow, |slice| slice.split_at(2));
1591 /// assert_eq!(*begin, [1, 2]);
1592 /// assert_eq!(*end, [3, 4]);
1593 /// ```
1594 #[stable(feature = "refcell_map_split", since = "1.35.0")]
1595 #[inline]
1596 pub fn map_split<U: ?Sized, V: ?Sized, F>(orig: Ref<'b, T>, f: F) -> (Ref<'b, U>, Ref<'b, V>)
1597 where
1598 F: FnOnce(&T) -> (&U, &V),
1599 {
1600 let (a, b) = f(&*orig);
1601 let borrow = orig.borrow.clone();
1602 (
1603 Ref { value: NonNull::from(a), borrow },
1604 Ref { value: NonNull::from(b), borrow: orig.borrow },
1605 )
1606 }
1607
1608 /// Converts into a reference to the underlying data.
1609 ///
1610 /// The underlying `RefCell` can never be mutably borrowed from again and will always appear
1611 /// already immutably borrowed. It is not a good idea to leak more than a constant number of
1612 /// references. The `RefCell` can be immutably borrowed again if only a smaller number of leaks
1613 /// have occurred in total.
1614 ///
1615 /// This is an associated function that needs to be used as
1616 /// `Ref::leak(...)`. A method would interfere with methods of the
1617 /// same name on the contents of a `RefCell` used through `Deref`.
1618 ///
1619 /// # Examples
1620 ///
1621 /// ```
1622 /// #![feature(cell_leak)]
1623 /// use std::cell::{RefCell, Ref};
1624 /// let cell = RefCell::new(0);
1625 ///
1626 /// let value = Ref::leak(cell.borrow());
1627 /// assert_eq!(*value, 0);
1628 ///
1629 /// assert!(cell.try_borrow().is_ok());
1630 /// assert!(cell.try_borrow_mut().is_err());
1631 /// ```
1632 #[unstable(feature = "cell_leak", issue = "69099")]
1633 #[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
1634 pub const fn leak(orig: Ref<'b, T>) -> &'b T {
1635 // By forgetting this Ref we ensure that the borrow counter in the RefCell can't go back to
1636 // UNUSED within the lifetime `'b`. Resetting the reference tracking state would require a
1637 // unique reference to the borrowed RefCell. No further mutable references can be created
1638 // from the original cell.
1639 mem::forget(orig.borrow);
1640 // SAFETY: after forgetting, we can form a reference for the rest of lifetime `'b`.
1641 unsafe { orig.value.as_ref() }
1642 }
1643}
1644
1645#[unstable(feature = "coerce_unsized", issue = "18598")]
1646impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Ref<'b, U>> for Ref<'b, T> {}
1647
1648#[stable(feature = "std_guard_impls", since = "1.20.0")]
1649impl<T: ?Sized + fmt::Display> fmt::Display for Ref<'_, T> {
1650 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1651 (**self).fmt(f)
1652 }
1653}
1654
1655impl<'b, T: ?Sized> RefMut<'b, T> {
1656 /// Makes a new `RefMut` for a component of the borrowed data, e.g., an enum
1657 /// variant.
1658 ///
1659 /// The `RefCell` is already mutably borrowed, so this cannot fail.
1660 ///
1661 /// This is an associated function that needs to be used as
1662 /// `RefMut::map(...)`. A method would interfere with methods of the same
1663 /// name on the contents of a `RefCell` used through `Deref`.
1664 ///
1665 /// # Examples
1666 ///
1667 /// ```
1668 /// use std::cell::{RefCell, RefMut};
1669 ///
1670 /// let c = RefCell::new((5, 'b'));
1671 /// {
1672 /// let b1: RefMut<'_, (u32, char)> = c.borrow_mut();
1673 /// let mut b2: RefMut<'_, u32> = RefMut::map(b1, |t| &mut t.0);
1674 /// assert_eq!(*b2, 5);
1675 /// *b2 = 42;
1676 /// }
1677 /// assert_eq!(*c.borrow(), (42, 'b'));
1678 /// ```
1679 #[stable(feature = "cell_map", since = "1.8.0")]
1680 #[inline]
1681 pub fn map<U: ?Sized, F>(mut orig: RefMut<'b, T>, f: F) -> RefMut<'b, U>
1682 where
1683 F: FnOnce(&mut T) -> &mut U,
1684 {
1685 let value = NonNull::from(f(&mut *orig));
1686 RefMut { value, borrow: orig.borrow, marker: PhantomData }
1687 }
1688
1689 /// Makes a new `RefMut` for an optional component of the borrowed data. The
1690 /// original guard is returned as an `Err(..)` if the closure returns
1691 /// `None`.
1692 ///
1693 /// The `RefCell` is already mutably borrowed, so this cannot fail.
1694 ///
1695 /// This is an associated function that needs to be used as
1696 /// `RefMut::filter_map(...)`. A method would interfere with methods of the
1697 /// same name on the contents of a `RefCell` used through `Deref`.
1698 ///
1699 /// # Examples
1700 ///
1701 /// ```
1702 /// use std::cell::{RefCell, RefMut};
1703 ///
1704 /// let c = RefCell::new(vec![1, 2, 3]);
1705 ///
1706 /// {
1707 /// let b1: RefMut<'_, Vec<u32>> = c.borrow_mut();
1708 /// let mut b2: Result<RefMut<'_, u32>, _> = RefMut::filter_map(b1, |v| v.get_mut(1));
1709 ///
1710 /// if let Ok(mut b2) = b2 {
1711 /// *b2 += 2;
1712 /// }
1713 /// }
1714 ///
1715 /// assert_eq!(*c.borrow(), vec![1, 4, 3]);
1716 /// ```
1717 #[stable(feature = "cell_filter_map", since = "1.63.0")]
1718 #[inline]
1719 pub fn filter_map<U: ?Sized, F>(mut orig: RefMut<'b, T>, f: F) -> Result<RefMut<'b, U>, Self>
1720 where
1721 F: FnOnce(&mut T) -> Option<&mut U>,
1722 {
1723 // SAFETY: function holds onto an exclusive reference for the duration
1724 // of its call through `orig`, and the pointer is only de-referenced
1725 // inside of the function call never allowing the exclusive reference to
1726 // escape.
1727 match f(&mut *orig) {
1728 Some(value) => {
1729 Ok(RefMut { value: NonNull::from(value), borrow: orig.borrow, marker: PhantomData })
1730 }
1731 None => Err(orig),
1732 }
1733 }
1734
1735 /// Splits a `RefMut` into multiple `RefMut`s for different components of the
1736 /// borrowed data.
1737 ///
1738 /// The underlying `RefCell` will remain mutably borrowed until both
1739 /// returned `RefMut`s go out of scope.
1740 ///
1741 /// The `RefCell` is already mutably borrowed, so this cannot fail.
1742 ///
1743 /// This is an associated function that needs to be used as
1744 /// `RefMut::map_split(...)`. A method would interfere with methods of the
1745 /// same name on the contents of a `RefCell` used through `Deref`.
1746 ///
1747 /// # Examples
1748 ///
1749 /// ```
1750 /// use std::cell::{RefCell, RefMut};
1751 ///
1752 /// let cell = RefCell::new([1, 2, 3, 4]);
1753 /// let borrow = cell.borrow_mut();
1754 /// let (mut begin, mut end) = RefMut::map_split(borrow, |slice| slice.split_at_mut(2));
1755 /// assert_eq!(*begin, [1, 2]);
1756 /// assert_eq!(*end, [3, 4]);
1757 /// begin.copy_from_slice(&[4, 3]);
1758 /// end.copy_from_slice(&[2, 1]);
1759 /// ```
1760 #[stable(feature = "refcell_map_split", since = "1.35.0")]
1761 #[inline]
1762 pub fn map_split<U: ?Sized, V: ?Sized, F>(
1763 mut orig: RefMut<'b, T>,
1764 f: F,
1765 ) -> (RefMut<'b, U>, RefMut<'b, V>)
1766 where
1767 F: FnOnce(&mut T) -> (&mut U, &mut V),
1768 {
1769 let borrow = orig.borrow.clone();
1770 let (a, b) = f(&mut *orig);
1771 (
1772 RefMut { value: NonNull::from(a), borrow, marker: PhantomData },
1773 RefMut { value: NonNull::from(b), borrow: orig.borrow, marker: PhantomData },
1774 )
1775 }
1776
1777 /// Converts into a mutable reference to the underlying data.
1778 ///
1779 /// The underlying `RefCell` can not be borrowed from again and will always appear already
1780 /// mutably borrowed, making the returned reference the only to the interior.
1781 ///
1782 /// This is an associated function that needs to be used as
1783 /// `RefMut::leak(...)`. A method would interfere with methods of the
1784 /// same name on the contents of a `RefCell` used through `Deref`.
1785 ///
1786 /// # Examples
1787 ///
1788 /// ```
1789 /// #![feature(cell_leak)]
1790 /// use std::cell::{RefCell, RefMut};
1791 /// let cell = RefCell::new(0);
1792 ///
1793 /// let value = RefMut::leak(cell.borrow_mut());
1794 /// assert_eq!(*value, 0);
1795 /// *value = 1;
1796 ///
1797 /// assert!(cell.try_borrow_mut().is_err());
1798 /// ```
1799 #[unstable(feature = "cell_leak", issue = "69099")]
1800 #[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
1801 pub const fn leak(mut orig: RefMut<'b, T>) -> &'b mut T {
1802 // By forgetting this BorrowRefMut we ensure that the borrow counter in the RefCell can't
1803 // go back to UNUSED within the lifetime `'b`. Resetting the reference tracking state would
1804 // require a unique reference to the borrowed RefCell. No further references can be created
1805 // from the original cell within that lifetime, making the current borrow the only
1806 // reference for the remaining lifetime.
1807 mem::forget(orig.borrow);
1808 // SAFETY: after forgetting, we can form a reference for the rest of lifetime `'b`.
1809 unsafe { orig.value.as_mut() }
1810 }
1811}
1812
1813struct BorrowRefMut<'b> {
1814 borrow: &'b Cell<BorrowCounter>,
1815}
1816
1817#[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
1818impl const Drop for BorrowRefMut<'_> {
1819 #[inline]
1820 fn drop(&mut self) {
1821 let borrow = self.borrow.get();
1822 debug_assert!(is_writing(borrow));
1823 self.borrow.replace(borrow + 1);
1824 }
1825}
1826
1827impl<'b> BorrowRefMut<'b> {
1828 #[inline]
1829 const fn new(borrow: &'b Cell<BorrowCounter>) -> Option<BorrowRefMut<'b>> {
1830 // NOTE: Unlike BorrowRefMut::clone, new is called to create the initial
1831 // mutable reference, and so there must currently be no existing
1832 // references. Thus, while clone increments the mutable refcount, here
1833 // we explicitly only allow going from UNUSED to UNUSED - 1.
1834 match borrow.get() {
1835 UNUSED => {
1836 borrow.replace(UNUSED - 1);
1837 Some(BorrowRefMut { borrow })
1838 }
1839 _ => None,
1840 }
1841 }
1842
1843 // Clones a `BorrowRefMut`.
1844 //
1845 // This is only valid if each `BorrowRefMut` is used to track a mutable
1846 // reference to a distinct, nonoverlapping range of the original object.
1847 // This isn't in a Clone impl so that code doesn't call this implicitly.
1848 #[inline]
1849 fn clone(&self) -> BorrowRefMut<'b> {
1850 let borrow = self.borrow.get();
1851 debug_assert!(is_writing(borrow));
1852 // Prevent the borrow counter from underflowing.
1853 assert!(borrow != BorrowCounter::MIN);
1854 self.borrow.set(borrow - 1);
1855 BorrowRefMut { borrow: self.borrow }
1856 }
1857}
1858
1859/// A wrapper type for a mutably borrowed value from a `RefCell<T>`.
1860///
1861/// See the [module-level documentation](self) for more.
1862#[stable(feature = "rust1", since = "1.0.0")]
1863#[must_not_suspend = "holding a RefMut across suspend points can cause BorrowErrors"]
1864#[rustc_diagnostic_item = "RefCellRefMut"]
1865pub struct RefMut<'b, T: ?Sized + 'b> {
1866 // NB: we use a pointer instead of `&'b mut T` to avoid `noalias` violations, because a
1867 // `RefMut` argument doesn't hold exclusivity for its whole scope, only until it drops.
1868 value: NonNull<T>,
1869 borrow: BorrowRefMut<'b>,
1870 // `NonNull` is covariant over `T`, so we need to reintroduce invariance.
1871 marker: PhantomData<&'b mut T>,
1872}
1873
1874#[stable(feature = "rust1", since = "1.0.0")]
1875#[rustc_const_unstable(feature = "const_deref", issue = "88955")]
1876impl<T: ?Sized> const Deref for RefMut<'_, T> {
1877 type Target = T;
1878
1879 #[inline]
1880 fn deref(&self) -> &T {
1881 // SAFETY: the value is accessible as long as we hold our borrow.
1882 unsafe { self.value.as_ref() }
1883 }
1884}
1885
1886#[stable(feature = "rust1", since = "1.0.0")]
1887#[rustc_const_unstable(feature = "const_deref", issue = "88955")]
1888impl<T: ?Sized> const DerefMut for RefMut<'_, T> {
1889 #[inline]
1890 fn deref_mut(&mut self) -> &mut T {
1891 // SAFETY: the value is accessible as long as we hold our borrow.
1892 unsafe { self.value.as_mut() }
1893 }
1894}
1895
1896#[unstable(feature = "deref_pure_trait", issue = "87121")]
1897unsafe impl<T: ?Sized> DerefPure for RefMut<'_, T> {}
1898
1899#[unstable(feature = "coerce_unsized", issue = "18598")]
1900impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<RefMut<'b, U>> for RefMut<'b, T> {}
1901
1902#[stable(feature = "std_guard_impls", since = "1.20.0")]
1903impl<T: ?Sized + fmt::Display> fmt::Display for RefMut<'_, T> {
1904 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1905 (**self).fmt(f)
1906 }
1907}
1908
1909/// The core primitive for interior mutability in Rust.
1910///
1911/// If you have a reference `&T`, then normally in Rust the compiler performs optimizations based on
1912/// the knowledge that `&T` points to immutable data. Mutating that data, for example through an
1913/// alias or by transmuting a `&T` into a `&mut T`, is considered undefined behavior.
1914/// `UnsafeCell<T>` opts-out of the immutability guarantee for `&T`: a shared reference
1915/// `&UnsafeCell<T>` may point to data that is being mutated. This is called "interior mutability".
1916///
1917/// All other types that allow internal mutability, such as [`Cell<T>`] and [`RefCell<T>`], internally
1918/// use `UnsafeCell` to wrap their data.
1919///
1920/// Note that only the immutability guarantee for shared references is affected by `UnsafeCell`. The
1921/// uniqueness guarantee for mutable references is unaffected. There is *no* legal way to obtain
1922/// aliasing `&mut`, not even with `UnsafeCell<T>`.
1923///
1924/// `UnsafeCell` does nothing to avoid data races; they are still undefined behavior. If multiple
1925/// threads have access to the same `UnsafeCell`, they must follow the usual rules of the
1926/// [concurrent memory model]: conflicting non-synchronized accesses must be done via the APIs in
1927/// [`core::sync::atomic`].
1928///
1929/// The `UnsafeCell` API itself is technically very simple: [`.get()`] gives you a raw pointer
1930/// `*mut T` to its contents. It is up to _you_ as the abstraction designer to use that raw pointer
1931/// correctly.
1932///
1933/// [`.get()`]: `UnsafeCell::get`
1934/// [concurrent memory model]: ../sync/atomic/index.html#memory-model-for-atomic-accesses
1935///
1936/// # Aliasing rules
1937///
1938/// The precise Rust aliasing rules are somewhat in flux, but the main points are not contentious:
1939///
1940/// - If you create a safe reference with lifetime `'a` (either a `&T` or `&mut T` reference), then
1941/// you must not access the data in any way that contradicts that reference for the remainder of
1942/// `'a`. For example, this means that if you take the `*mut T` from an `UnsafeCell<T>` and cast it
1943/// to an `&T`, then the data in `T` must remain immutable (modulo any `UnsafeCell` data found
1944/// within `T`, of course) until that reference's lifetime expires. Similarly, if you create a `&mut
1945/// T` reference that is released to safe code, then you must not access the data within the
1946/// `UnsafeCell` until that reference expires.
1947///
1948/// - For both `&T` without `UnsafeCell<_>` and `&mut T`, you must also not deallocate the data
1949/// until the reference expires. As a special exception, given an `&T`, any part of it that is
1950/// inside an `UnsafeCell<_>` may be deallocated during the lifetime of the reference, after the
1951/// last time the reference is used (dereferenced or reborrowed). Since you cannot deallocate a part
1952/// of what a reference points to, this means the memory an `&T` points to can be deallocated only if
1953/// *every part of it* (including padding) is inside an `UnsafeCell`.
1954///
1955/// However, whenever a `&UnsafeCell<T>` is constructed or dereferenced, it must still point to
1956/// live memory and the compiler is allowed to insert spurious reads if it can prove that this
1957/// memory has not yet been deallocated.
1958///
1959/// To assist with proper design, the following scenarios are explicitly declared legal
1960/// for single-threaded code:
1961///
1962/// 1. A `&T` reference can be released to safe code and there it can co-exist with other `&T`
1963/// references, but not with a `&mut T`
1964///
1965/// 2. A `&mut T` reference may be released to safe code provided neither other `&mut T` nor `&T`
1966/// co-exist with it. A `&mut T` must always be unique.
1967///
1968/// Note that whilst mutating the contents of an `&UnsafeCell<T>` (even while other
1969/// `&UnsafeCell<T>` references alias the cell) is
1970/// ok (provided you enforce the above invariants some other way), it is still undefined behavior
1971/// to have multiple `&mut UnsafeCell<T>` aliases. That is, `UnsafeCell` is a wrapper
1972/// designed to have a special interaction with _shared_ accesses (_i.e._, through an
1973/// `&UnsafeCell<_>` reference); there is no magic whatsoever when dealing with _exclusive_
1974/// accesses (_e.g._, through a `&mut UnsafeCell<_>`): neither the cell nor the wrapped value
1975/// may be aliased for the duration of that `&mut` borrow.
1976/// This is showcased by the [`.get_mut()`] accessor, which is a _safe_ getter that yields
1977/// a `&mut T`.
1978///
1979/// [`.get_mut()`]: `UnsafeCell::get_mut`
1980///
1981/// # Memory layout
1982///
1983/// `UnsafeCell<T>` has the same in-memory representation as its inner type `T`. A consequence
1984/// of this guarantee is that it is possible to convert between `T` and `UnsafeCell<T>`.
1985/// Special care has to be taken when converting a nested `T` inside of an `Outer<T>` type
1986/// to an `Outer<UnsafeCell<T>>` type: this is not sound when the `Outer<T>` type enables [niche]
1987/// optimizations. For example, the type `Option<NonNull<u8>>` is typically 8 bytes large on
1988/// 64-bit platforms, but the type `Option<UnsafeCell<NonNull<u8>>>` takes up 16 bytes of space.
1989/// Therefore this is not a valid conversion, despite `NonNull<u8>` and `UnsafeCell<NonNull<u8>>>`
1990/// having the same memory layout. This is because `UnsafeCell` disables niche optimizations in
1991/// order to avoid its interior mutability property from spreading from `T` into the `Outer` type,
1992/// thus this can cause distortions in the type size in these cases.
1993///
1994/// Note that the only valid way to obtain a `*mut T` pointer to the contents of a
1995/// _shared_ `UnsafeCell<T>` is through [`.get()`] or [`.raw_get()`]. A `&mut T` reference
1996/// can be obtained by either dereferencing this pointer or by calling [`.get_mut()`]
1997/// on an _exclusive_ `UnsafeCell<T>`. Even though `T` and `UnsafeCell<T>` have the
1998/// same memory layout, the following is not allowed and undefined behavior:
1999///
2000/// ```rust,compile_fail
2001/// # use std::cell::UnsafeCell;
2002/// unsafe fn not_allowed<T>(ptr: &UnsafeCell<T>) -> &mut T {
2003/// let t = ptr as *const UnsafeCell<T> as *mut T;
2004/// // This is undefined behavior, because the `*mut T` pointer
2005/// // was not obtained through `.get()` nor `.raw_get()`:
2006/// unsafe { &mut *t }
2007/// }
2008/// ```
2009///
2010/// Instead, do this:
2011///
2012/// ```rust
2013/// # use std::cell::UnsafeCell;
2014/// // Safety: the caller must ensure that there are no references that
2015/// // point to the *contents* of the `UnsafeCell`.
2016/// unsafe fn get_mut<T>(ptr: &UnsafeCell<T>) -> &mut T {
2017/// unsafe { &mut *ptr.get() }
2018/// }
2019/// ```
2020///
2021/// Converting in the other direction from a `&mut T`
2022/// to an `&UnsafeCell<T>` is allowed:
2023///
2024/// ```rust
2025/// # use std::cell::UnsafeCell;
2026/// fn get_shared<T>(ptr: &mut T) -> &UnsafeCell<T> {
2027/// let t = ptr as *mut T as *const UnsafeCell<T>;
2028/// // SAFETY: `T` and `UnsafeCell<T>` have the same memory layout
2029/// unsafe { &*t }
2030/// }
2031/// ```
2032///
2033/// [niche]: https://rust-lang.github.io/unsafe-code-guidelines/glossary.html#niche
2034/// [`.raw_get()`]: `UnsafeCell::raw_get`
2035///
2036/// # Examples
2037///
2038/// Here is an example showcasing how to soundly mutate the contents of an `UnsafeCell<_>` despite
2039/// there being multiple references aliasing the cell:
2040///
2041/// ```
2042/// use std::cell::UnsafeCell;
2043///
2044/// let x: UnsafeCell<i32> = 42.into();
2045/// // Get multiple / concurrent / shared references to the same `x`.
2046/// let (p1, p2): (&UnsafeCell<i32>, &UnsafeCell<i32>) = (&x, &x);
2047///
2048/// unsafe {
2049/// // SAFETY: within this scope there are no other references to `x`'s contents,
2050/// // so ours is effectively unique.
2051/// let p1_exclusive: &mut i32 = &mut *p1.get(); // -- borrow --+
2052/// *p1_exclusive += 27; // |
2053/// } // <---------- cannot go beyond this point -------------------+
2054///
2055/// unsafe {
2056/// // SAFETY: within this scope nobody expects to have exclusive access to `x`'s contents,
2057/// // so we can have multiple shared accesses concurrently.
2058/// let p2_shared: &i32 = &*p2.get();
2059/// assert_eq!(*p2_shared, 42 + 27);
2060/// let p1_shared: &i32 = &*p1.get();
2061/// assert_eq!(*p1_shared, *p2_shared);
2062/// }
2063/// ```
2064///
2065/// The following example showcases the fact that exclusive access to an `UnsafeCell<T>`
2066/// implies exclusive access to its `T`:
2067///
2068/// ```rust
2069/// #![forbid(unsafe_code)] // with exclusive accesses,
2070/// // `UnsafeCell` is a transparent no-op wrapper,
2071/// // so no need for `unsafe` here.
2072/// use std::cell::UnsafeCell;
2073///
2074/// let mut x: UnsafeCell<i32> = 42.into();
2075///
2076/// // Get a compile-time-checked unique reference to `x`.
2077/// let p_unique: &mut UnsafeCell<i32> = &mut x;
2078/// // With an exclusive reference, we can mutate the contents for free.
2079/// *p_unique.get_mut() = 0;
2080/// // Or, equivalently:
2081/// x = UnsafeCell::new(0);
2082///
2083/// // When we own the value, we can extract the contents for free.
2084/// let contents: i32 = x.into_inner();
2085/// assert_eq!(contents, 0);
2086/// ```
2087#[lang = "unsafe_cell"]
2088#[stable(feature = "rust1", since = "1.0.0")]
2089#[repr(transparent)]
2090#[rustc_pub_transparent]
2091pub struct UnsafeCell<T: ?Sized> {
2092 value: T,
2093}
2094
2095#[stable(feature = "rust1", since = "1.0.0")]
2096impl<T: ?Sized> !Sync for UnsafeCell<T> {}
2097
2098impl<T> UnsafeCell<T> {
2099 /// Constructs a new instance of `UnsafeCell` which will wrap the specified
2100 /// value.
2101 ///
2102 /// All access to the inner value through `&UnsafeCell<T>` requires `unsafe` code.
2103 ///
2104 /// # Examples
2105 ///
2106 /// ```
2107 /// use std::cell::UnsafeCell;
2108 ///
2109 /// let uc = UnsafeCell::new(5);
2110 /// ```
2111 #[stable(feature = "rust1", since = "1.0.0")]
2112 #[rustc_const_stable(feature = "const_unsafe_cell_new", since = "1.32.0")]
2113 #[inline(always)]
2114 pub const fn new(value: T) -> UnsafeCell<T> {
2115 UnsafeCell { value }
2116 }
2117
2118 /// Unwraps the value, consuming the cell.
2119 ///
2120 /// # Examples
2121 ///
2122 /// ```
2123 /// use std::cell::UnsafeCell;
2124 ///
2125 /// let uc = UnsafeCell::new(5);
2126 ///
2127 /// let five = uc.into_inner();
2128 /// ```
2129 #[inline(always)]
2130 #[stable(feature = "rust1", since = "1.0.0")]
2131 #[rustc_const_stable(feature = "const_cell_into_inner", since = "1.83.0")]
2132 #[rustc_allow_const_fn_unstable(const_precise_live_drops)]
2133 pub const fn into_inner(self) -> T {
2134 self.value
2135 }
2136
2137 /// Replace the value in this `UnsafeCell` and return the old value.
2138 ///
2139 /// # Safety
2140 ///
2141 /// The caller must take care to avoid aliasing and data races.
2142 ///
2143 /// - It is Undefined Behavior to allow calls to race with
2144 /// any other access to the wrapped value.
2145 /// - It is Undefined Behavior to call this while any other
2146 /// reference(s) to the wrapped value are alive.
2147 ///
2148 /// # Examples
2149 ///
2150 /// ```
2151 /// #![feature(unsafe_cell_access)]
2152 /// use std::cell::UnsafeCell;
2153 ///
2154 /// let uc = UnsafeCell::new(5);
2155 ///
2156 /// let old = unsafe { uc.replace(10) };
2157 /// assert_eq!(old, 5);
2158 /// ```
2159 #[inline]
2160 #[unstable(feature = "unsafe_cell_access", issue = "136327")]
2161 pub const unsafe fn replace(&self, value: T) -> T {
2162 // SAFETY: pointer comes from `&self` so naturally satisfies invariants.
2163 unsafe { ptr::replace(self.get(), value) }
2164 }
2165}
2166
2167impl<T: ?Sized> UnsafeCell<T> {
2168 /// Converts from `&mut T` to `&mut UnsafeCell<T>`.
2169 ///
2170 /// # Examples
2171 ///
2172 /// ```
2173 /// use std::cell::UnsafeCell;
2174 ///
2175 /// let mut val = 42;
2176 /// let uc = UnsafeCell::from_mut(&mut val);
2177 ///
2178 /// *uc.get_mut() -= 1;
2179 /// assert_eq!(*uc.get_mut(), 41);
2180 /// ```
2181 #[inline(always)]
2182 #[stable(feature = "unsafe_cell_from_mut", since = "1.84.0")]
2183 #[rustc_const_stable(feature = "unsafe_cell_from_mut", since = "1.84.0")]
2184 pub const fn from_mut(value: &mut T) -> &mut UnsafeCell<T> {
2185 // SAFETY: `UnsafeCell<T>` has the same memory layout as `T` due to #[repr(transparent)].
2186 unsafe { &mut *(value as *mut T as *mut UnsafeCell<T>) }
2187 }
2188
2189 /// Gets a mutable pointer to the wrapped value.
2190 ///
2191 /// This can be cast to a pointer of any kind. When creating references, you must uphold the
2192 /// aliasing rules; see [the type-level docs][UnsafeCell#aliasing-rules] for more discussion and
2193 /// caveats.
2194 ///
2195 /// # Examples
2196 ///
2197 /// ```
2198 /// use std::cell::UnsafeCell;
2199 ///
2200 /// let uc = UnsafeCell::new(5);
2201 ///
2202 /// let five = uc.get();
2203 /// ```
2204 #[inline(always)]
2205 #[stable(feature = "rust1", since = "1.0.0")]
2206 #[rustc_const_stable(feature = "const_unsafecell_get", since = "1.32.0")]
2207 #[rustc_as_ptr]
2208 #[rustc_never_returns_null_ptr]
2209 pub const fn get(&self) -> *mut T {
2210 // We can just cast the pointer from `UnsafeCell<T>` to `T` because of
2211 // #[repr(transparent)]. This exploits std's special status, there is
2212 // no guarantee for user code that this will work in future versions of the compiler!
2213 self as *const UnsafeCell<T> as *const T as *mut T
2214 }
2215
2216 /// Returns a mutable reference to the underlying data.
2217 ///
2218 /// This call borrows the `UnsafeCell` mutably (at compile-time) which
2219 /// guarantees that we possess the only reference.
2220 ///
2221 /// # Examples
2222 ///
2223 /// ```
2224 /// use std::cell::UnsafeCell;
2225 ///
2226 /// let mut c = UnsafeCell::new(5);
2227 /// *c.get_mut() += 1;
2228 ///
2229 /// assert_eq!(*c.get_mut(), 6);
2230 /// ```
2231 #[inline(always)]
2232 #[stable(feature = "unsafe_cell_get_mut", since = "1.50.0")]
2233 #[rustc_const_stable(feature = "const_unsafecell_get_mut", since = "1.83.0")]
2234 pub const fn get_mut(&mut self) -> &mut T {
2235 &mut self.value
2236 }
2237
2238 /// Gets a mutable pointer to the wrapped value.
2239 /// The difference from [`get`] is that this function accepts a raw pointer,
2240 /// which is useful to avoid the creation of temporary references.
2241 ///
2242 /// This can be cast to a pointer of any kind. When creating references, you must uphold the
2243 /// aliasing rules; see [the type-level docs][UnsafeCell#aliasing-rules] for more discussion and
2244 /// caveats.
2245 ///
2246 /// [`get`]: UnsafeCell::get()
2247 ///
2248 /// # Examples
2249 ///
2250 /// Gradual initialization of an `UnsafeCell` requires `raw_get`, as
2251 /// calling `get` would require creating a reference to uninitialized data:
2252 ///
2253 /// ```
2254 /// use std::cell::UnsafeCell;
2255 /// use std::mem::MaybeUninit;
2256 ///
2257 /// let m = MaybeUninit::<UnsafeCell<i32>>::uninit();
2258 /// unsafe { UnsafeCell::raw_get(m.as_ptr()).write(5); }
2259 /// // avoid below which references to uninitialized data
2260 /// // unsafe { UnsafeCell::get(&*m.as_ptr()).write(5); }
2261 /// let uc = unsafe { m.assume_init() };
2262 ///
2263 /// assert_eq!(uc.into_inner(), 5);
2264 /// ```
2265 #[inline(always)]
2266 #[stable(feature = "unsafe_cell_raw_get", since = "1.56.0")]
2267 #[rustc_const_stable(feature = "unsafe_cell_raw_get", since = "1.56.0")]
2268 #[rustc_diagnostic_item = "unsafe_cell_raw_get"]
2269 pub const fn raw_get(this: *const Self) -> *mut T {
2270 // We can just cast the pointer from `UnsafeCell<T>` to `T` because of
2271 // #[repr(transparent)]. This exploits std's special status, there is
2272 // no guarantee for user code that this will work in future versions of the compiler!
2273 this as *const T as *mut T
2274 }
2275
2276 /// Get a shared reference to the value within the `UnsafeCell`.
2277 ///
2278 /// # Safety
2279 ///
2280 /// - It is Undefined Behavior to call this while any mutable
2281 /// reference to the wrapped value is alive.
2282 /// - Mutating the wrapped value while the returned
2283 /// reference is alive is Undefined Behavior.
2284 ///
2285 /// # Examples
2286 ///
2287 /// ```
2288 /// #![feature(unsafe_cell_access)]
2289 /// use std::cell::UnsafeCell;
2290 ///
2291 /// let uc = UnsafeCell::new(5);
2292 ///
2293 /// let val = unsafe { uc.as_ref_unchecked() };
2294 /// assert_eq!(val, &5);
2295 /// ```
2296 #[inline]
2297 #[unstable(feature = "unsafe_cell_access", issue = "136327")]
2298 pub const unsafe fn as_ref_unchecked(&self) -> &T {
2299 // SAFETY: pointer comes from `&self` so naturally satisfies ptr-to-ref invariants.
2300 unsafe { self.get().as_ref_unchecked() }
2301 }
2302
2303 /// Get an exclusive reference to the value within the `UnsafeCell`.
2304 ///
2305 /// # Safety
2306 ///
2307 /// - It is Undefined Behavior to call this while any other
2308 /// reference(s) to the wrapped value are alive.
2309 /// - Mutating the wrapped value through other means while the
2310 /// returned reference is alive is Undefined Behavior.
2311 ///
2312 /// # Examples
2313 ///
2314 /// ```
2315 /// #![feature(unsafe_cell_access)]
2316 /// use std::cell::UnsafeCell;
2317 ///
2318 /// let uc = UnsafeCell::new(5);
2319 ///
2320 /// unsafe { *uc.as_mut_unchecked() += 1; }
2321 /// assert_eq!(uc.into_inner(), 6);
2322 /// ```
2323 #[inline]
2324 #[unstable(feature = "unsafe_cell_access", issue = "136327")]
2325 #[allow(clippy::mut_from_ref)]
2326 pub const unsafe fn as_mut_unchecked(&self) -> &mut T {
2327 // SAFETY: pointer comes from `&self` so naturally satisfies ptr-to-ref invariants.
2328 unsafe { self.get().as_mut_unchecked() }
2329 }
2330}
2331
2332#[stable(feature = "unsafe_cell_default", since = "1.10.0")]
2333impl<T: Default> Default for UnsafeCell<T> {
2334 /// Creates an `UnsafeCell`, with the `Default` value for T.
2335 fn default() -> UnsafeCell<T> {
2336 UnsafeCell::new(Default::default())
2337 }
2338}
2339
2340#[stable(feature = "cell_from", since = "1.12.0")]
2341impl<T> From<T> for UnsafeCell<T> {
2342 /// Creates a new `UnsafeCell<T>` containing the given value.
2343 fn from(t: T) -> UnsafeCell<T> {
2344 UnsafeCell::new(t)
2345 }
2346}
2347
2348#[unstable(feature = "coerce_unsized", issue = "18598")]
2349impl<T: CoerceUnsized<U>, U> CoerceUnsized<UnsafeCell<U>> for UnsafeCell<T> {}
2350
2351// Allow types that wrap `UnsafeCell` to also implement `DispatchFromDyn`
2352// and become dyn-compatible method receivers.
2353// Note that currently `UnsafeCell` itself cannot be a method receiver
2354// because it does not implement Deref.
2355// In other words:
2356// `self: UnsafeCell<&Self>` won't work
2357// `self: UnsafeCellWrapper<Self>` becomes possible
2358#[unstable(feature = "dispatch_from_dyn", issue = "none")]
2359impl<T: DispatchFromDyn<U>, U> DispatchFromDyn<UnsafeCell<U>> for UnsafeCell<T> {}
2360
2361/// [`UnsafeCell`], but [`Sync`].
2362///
2363/// This is just an `UnsafeCell`, except it implements `Sync`
2364/// if `T` implements `Sync`.
2365///
2366/// `UnsafeCell` doesn't implement `Sync`, to prevent accidental mis-use.
2367/// You can use `SyncUnsafeCell` instead of `UnsafeCell` to allow it to be
2368/// shared between threads, if that's intentional.
2369/// Providing proper synchronization is still the task of the user,
2370/// making this type just as unsafe to use.
2371///
2372/// See [`UnsafeCell`] for details.
2373#[unstable(feature = "sync_unsafe_cell", issue = "95439")]
2374#[repr(transparent)]
2375#[rustc_diagnostic_item = "SyncUnsafeCell"]
2376#[rustc_pub_transparent]
2377pub struct SyncUnsafeCell<T: ?Sized> {
2378 value: UnsafeCell<T>,
2379}
2380
2381#[unstable(feature = "sync_unsafe_cell", issue = "95439")]
2382unsafe impl<T: ?Sized + Sync> Sync for SyncUnsafeCell<T> {}
2383
2384#[unstable(feature = "sync_unsafe_cell", issue = "95439")]
2385impl<T> SyncUnsafeCell<T> {
2386 /// Constructs a new instance of `SyncUnsafeCell` which will wrap the specified value.
2387 #[inline]
2388 pub const fn new(value: T) -> Self {
2389 Self { value: UnsafeCell { value } }
2390 }
2391
2392 /// Unwraps the value, consuming the cell.
2393 #[inline]
2394 #[rustc_const_unstable(feature = "sync_unsafe_cell", issue = "95439")]
2395 pub const fn into_inner(self) -> T {
2396 self.value.into_inner()
2397 }
2398}
2399
2400#[unstable(feature = "sync_unsafe_cell", issue = "95439")]
2401impl<T: ?Sized> SyncUnsafeCell<T> {
2402 /// Gets a mutable pointer to the wrapped value.
2403 ///
2404 /// This can be cast to a pointer of any kind.
2405 /// Ensure that the access is unique (no active references, mutable or not)
2406 /// when casting to `&mut T`, and ensure that there are no mutations
2407 /// or mutable aliases going on when casting to `&T`
2408 #[inline]
2409 #[rustc_as_ptr]
2410 #[rustc_never_returns_null_ptr]
2411 pub const fn get(&self) -> *mut T {
2412 self.value.get()
2413 }
2414
2415 /// Returns a mutable reference to the underlying data.
2416 ///
2417 /// This call borrows the `SyncUnsafeCell` mutably (at compile-time) which
2418 /// guarantees that we possess the only reference.
2419 #[inline]
2420 pub const fn get_mut(&mut self) -> &mut T {
2421 self.value.get_mut()
2422 }
2423
2424 /// Gets a mutable pointer to the wrapped value.
2425 ///
2426 /// See [`UnsafeCell::get`] for details.
2427 #[inline]
2428 pub const fn raw_get(this: *const Self) -> *mut T {
2429 // We can just cast the pointer from `SyncUnsafeCell<T>` to `T` because
2430 // of #[repr(transparent)] on both SyncUnsafeCell and UnsafeCell.
2431 // See UnsafeCell::raw_get.
2432 this as *const T as *mut T
2433 }
2434}
2435
2436#[unstable(feature = "sync_unsafe_cell", issue = "95439")]
2437impl<T: Default> Default for SyncUnsafeCell<T> {
2438 /// Creates an `SyncUnsafeCell`, with the `Default` value for T.
2439 fn default() -> SyncUnsafeCell<T> {
2440 SyncUnsafeCell::new(Default::default())
2441 }
2442}
2443
2444#[unstable(feature = "sync_unsafe_cell", issue = "95439")]
2445impl<T> From<T> for SyncUnsafeCell<T> {
2446 /// Creates a new `SyncUnsafeCell<T>` containing the given value.
2447 fn from(t: T) -> SyncUnsafeCell<T> {
2448 SyncUnsafeCell::new(t)
2449 }
2450}
2451
2452#[unstable(feature = "coerce_unsized", issue = "18598")]
2453//#[unstable(feature = "sync_unsafe_cell", issue = "95439")]
2454impl<T: CoerceUnsized<U>, U> CoerceUnsized<SyncUnsafeCell<U>> for SyncUnsafeCell<T> {}
2455
2456// Allow types that wrap `SyncUnsafeCell` to also implement `DispatchFromDyn`
2457// and become dyn-compatible method receivers.
2458// Note that currently `SyncUnsafeCell` itself cannot be a method receiver
2459// because it does not implement Deref.
2460// In other words:
2461// `self: SyncUnsafeCell<&Self>` won't work
2462// `self: SyncUnsafeCellWrapper<Self>` becomes possible
2463#[unstable(feature = "dispatch_from_dyn", issue = "none")]
2464//#[unstable(feature = "sync_unsafe_cell", issue = "95439")]
2465impl<T: DispatchFromDyn<U>, U> DispatchFromDyn<SyncUnsafeCell<U>> for SyncUnsafeCell<T> {}
2466
2467#[allow(unused)]
2468fn assert_coerce_unsized(
2469 a: UnsafeCell<&i32>,
2470 b: SyncUnsafeCell<&i32>,
2471 c: Cell<&i32>,
2472 d: RefCell<&i32>,
2473) {
2474 let _: UnsafeCell<&dyn Send> = a;
2475 let _: SyncUnsafeCell<&dyn Send> = b;
2476 let _: Cell<&dyn Send> = c;
2477 let _: RefCell<&dyn Send> = d;
2478}
2479
2480#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
2481unsafe impl<T: ?Sized> PinCoerceUnsized for UnsafeCell<T> {}
2482
2483#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
2484unsafe impl<T: ?Sized> PinCoerceUnsized for SyncUnsafeCell<T> {}
2485
2486#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
2487unsafe impl<T: ?Sized> PinCoerceUnsized for Cell<T> {}
2488
2489#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
2490unsafe impl<T: ?Sized> PinCoerceUnsized for RefCell<T> {}
2491
2492#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
2493unsafe impl<'b, T: ?Sized> PinCoerceUnsized for Ref<'b, T> {}
2494
2495#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
2496unsafe impl<'b, T: ?Sized> PinCoerceUnsized for RefMut<'b, T> {}